Question 1

Which of the following substances is a pure chemical element rather than a mixture or compound: glass, cement or sodium?

Accepted Answer

Introduction / Context: This chemistry question checks your basic understanding of what is meant by a pure element, a compound and a mixture. Distinguishing between these types of substances is fundamental for topics such as chemical reactions, stoichiometry and material science. Given Data / Assumptions: Options include glass, cement, sodium, none of these and concrete. We are asked to identify which option represents a pure chemical element. We assume normal everyday meanings of these materials as used in school and competitive exam chemistry. Pure element means a substance made up of only one kind of atom. Concept / Approach: A chemical element is a substance that cannot be broken down into simpler substances by ordinary chemical processes and whose atoms all have the same number of protons. Compounds are formed when atoms of different elements chemically combine in fixed ratios. Mixtures consist of two or more substances physically combined, not necessarily in fixed proportions. Therefore, we identify whether each option is an element, a mixture or a compound, and then choose the pure element from the list. Step-by-Step Solution: 1) Glass is usually a mixture of silica (silicon dioxide) and other metal oxides; it is not a single pure compound or element.2) Cement is a complex mixture of compounds, mainly calcium silicates and aluminates, and again is not a pure element.3) Sodium is a metallic chemical element with symbol Na and atomic number 11 in the periodic table.4) Concrete is a mixture of cement, sand, gravel and water, so it is clearly not a pure element.5) Since sodium consists of only one type of atom, it is the only pure element among the given choices. Verification / Alternative check: We can check with periodic table knowledge. Sodium appears as an alkali metal in Group 1 and is a basic building block for many compounds such as sodium chloride. Glass, cement and concrete are not present as single entries in the periodic table because they are not pure elements. This confirms that sodium is the correct answer and satisfies the definition of a pure chemical element. Why Other Options Are Wrong: Glass: It is an amorphous mixture, often containing silica, lime and other oxides, so it does not qualify as a pure element. Cement: It is a complex mixture of many compounds such as calcium silicates, not a single pure substance or element. None of these: This would be correct only if no option were a pure element, but sodium clearly is one, so this choice is incorrect. Concrete: It is made by mixing cement, sand and aggregates with water and is a classic example of a heterogeneous mixture, not an element. Common Pitfalls: A common mistake is to confuse common construction materials like glass or cement with pure substances because they may appear uniform to the naked eye. However, uniform appearance does not guarantee chemical purity. Another trap is to choose none of these when unsure, instead of recalling that metals like sodium are listed individually in the periodic table as elements. Remember that any familiar metal such as sodium, copper or iron in its pure form is an element, whereas everyday building materials are usually mixtures or complex compounds. Final Answer: The pure chemical element among the given options is sodium.

Question 2

A laboratory mixture of sand and naphthalene can be separated effectively by using which separation technique?

Accepted Answer

Introduction / Context: This practical chemistry question tests your knowledge of basic separation techniques for mixtures of solids. Choosing the correct method depends on specific physical properties of the substances involved, such as volatility, solubility and sublimation behaviour. Given Data / Assumptions: The mixture consists of sand and naphthalene. Sand is an inorganic, non volatile solid that does not sublime. Naphthalene is an organic solid known to sublime on gentle heating. We are asked to identify the best method to separate these two substances. Concept / Approach: Sublimation is a process in which a solid changes directly to vapour without passing through a liquid state, and then condenses back to solid on cooling. Certain organic compounds like naphthalene, camphor and iodine readily sublime on heating, whereas sand does not. If one component of a mixture sublimes and the other does not, sublimation is the ideal separation technique. Distillation and chromatography are meant for different types of mixtures and are not efficient here. Step-by-Step Solution: 1) Recognise that naphthalene is a volatile solid that undergoes sublimation on gentle heating.2) Sand, mainly composed of silica, is non volatile at such temperatures and will not sublime.3) If the mixture is placed in a sublimation apparatus and heated, only naphthalene will vaporise and move as vapour.4) The vapours of naphthalene are cooled on a cold surface, where they condense and deposit as pure solid naphthalene crystals.5) Sand remains in the original container, so the two substances are effectively separated by sublimation. Verification / Alternative check: We can check the other methods. Fractional distillation is used for separating liquid mixtures based on boiling point differences, so it is not appropriate for a solid solid mixture like sand and naphthalene. Chromatography is generally used for separating components dissolved in a mobile phase, not for bulk solid mixtures of this type. Simple filtration alone would not work because both components are solids and are not in different phases. Therefore sublimation is clearly the most suitable technique in this scenario. Why Other Options Are Wrong: Fractional distillation: This technique requires both components to be liquids or vapourisable liquids; it is not suitable for solid solid mixtures like sand and naphthalene. Chromatography: Used mainly for separation of small amounts of components in solution; it is not a standard method for bulk separation of sand and naphthalene. Filtration only: Since neither sand nor naphthalene is dissolved in a liquid here, filtration does not separate them effectively. Crystallisation from hot water: Naphthalene is not very soluble in water and might not give clean separation using this method, whereas sublimation is straightforward and standard for this mixture. Common Pitfalls: Students sometimes choose distillation or filtration out of habit, without first identifying the special property of naphthalene, namely sublimation. Another misconception is to think that any solid mixture must be separated by sieving or filtration, but in reality the best method depends on properties like ability to sublime or differences in solubility. Always start such questions by listing key properties of each component and then match them with the most suitable separation technique. Final Answer: The mixture of sand and naphthalene can be separated most effectively by sublimation.

Question 3

In electrochemistry, what is the SI unit used for equivalent conductance (Λeq) of an electrolyte solution?

Accepted Answer

Introduction / Context: This question tests your knowledge of the correct SI unit for equivalent conductance, a quantity commonly used in electrochemistry to describe the conducting power of an electrolyte solution for one gram equivalent of solute. Correct units are important when you interpret experimental data or compare values from different sources. Given Data / Assumptions: We are dealing with equivalent conductance, usually denoted by Λeq or lambda eq. Options list different possible units like ohm/cm, S m2 per equivalent and others. We assume SI base units and standard definitions are being used. Conductance is the reciprocal of resistance and is measured in siemens (S) in SI units. Concept / Approach: Equivalent conductance Λeq is defined as the conductance of a solution containing one gram equivalent of electrolyte placed between electrodes that are one metre apart and have a cross sectional area large enough to contain that equivalent. Mathematically, Λeq is related to specific conductance k and equivalent concentration. In SI, specific conductance k has units of S per metre, and equivalent conductance scales this by the volume that contains one equivalent. This leads to units of siemens multiplied by square metre per equivalent, written as S m2 per equivalent. Step-by-Step Solution: 1) Specific conductance k has SI unit S per metre (S/m).2) Equivalent conductance Λeq is related to k by Λeq = k * V, where V is the volume that contains one gram equivalent of solute.3) If V is expressed in cubic metres per equivalent, then Λeq has units S/m * m3 per equivalent = S m2 per equivalent.4) Therefore, the correct SI unit for equivalent conductance is siemens square metre per equivalent, written as S m2 per equivalent.5) Older units like mho and centimetre based units belong to the CGS system and are not strict SI units. Verification / Alternative check: In older literature, equivalent conductance is often given in units of S cm2 per equivalent or previously mho cm2 per equivalent. To convert to SI, note that 1 cm = 0.01 m, so 1 cm2 = 10^-4 m2. Thus multiplying by cm2 per equivalent in CGS naturally corresponds to m2 per equivalent in SI after proper conversion. This is consistent with the expression S m2 per equivalent and confirms the reasoning based on dimensional analysis. Why Other Options Are Wrong: ohm/cm: This is a unit of resistivity, not conductance, and it is not in SI form. siemens per equivalent (S per equivalent): This ignores the squared length dimension that arises from converting specific conductance to equivalent conductance. mho/cm: This is an older CGS unit for specific conductance, not equivalent conductance and not SI compliant. ohm square metre per mole: This unit mixes resistance with mole based concentration and does not represent equivalent conductance of electrolyte solutions. Common Pitfalls: Students often confuse specific conductance, molar conductance and equivalent conductance, as well as their respective units. Another common error is to memorise CGS units using centimetres and mho and then assume they are SI units. To avoid confusion, always start from the definition of the quantity, express it in terms of basic SI units, and perform dimensional analysis. This approach will lead you reliably to the correct unit even if you forget the exact form from memory. Final Answer: The SI unit of equivalent conductance is siemens square metre per equivalent (S m2 per equivalent).

Question 4

Why was the discovery of the neutron made relatively late compared to the discovery of other subatomic particles such as the electron and proton?

Accepted Answer

Introduction / Context: This question belongs to atomic structure and modern physics. It asks why the neutron, discovered by James Chadwick in 1932, was identified significantly later than the electron and proton. Understanding this reason gives insight into how experimental detection of particles depends on their charge and interaction with fields. Given Data / Assumptions: We are comparing the discovery timeline of the neutron with that of the electron and proton. Options suggest different possible reasons, such as location in the nucleus or absence of charge. We assume basic knowledge that neutrons are neutral, while electrons and protons are charged. The question asks for the main reason for the delayed discovery. Concept / Approach: Charged particles like electrons and protons interact strongly with electric and magnetic fields, so they leave clear signatures in many experimental setups, such as deflection in electric and magnetic fields. Neutral particles like neutrons do not experience electromagnetic forces, so they cannot be detected easily by standard methods based on deflection or ionisation by direct charge. Hence, discovering the neutron required more indirect experimental strategies involving nuclear reactions and scattering. Step-by-Step Solution: 1) Electrons were discovered by J. J. Thomson using cathode ray experiments, which relied on the deflection of charged particles in electric and magnetic fields.2) Protons were identified as positively charged particles in canal rays, again detected due to their interaction with fields.3) Neutrons, on the other hand, have no electric charge, so they do not respond to electric fields and produce no direct ionisation trails in many detectors.4) Because of this neutrality, early experimental techniques could not easily detect or distinguish them from other radiation.5) Only when Chadwick carefully analysed nuclear bombardment experiments did he infer and confirm the existence of a neutral, massive particle, later named the neutron. Verification / Alternative check: Historical accounts show that scientists earlier suspected some neutral mass within the nucleus because atomic mass exceeded what could be explained by protons alone. However, there was no direct experimental evidence until neutron scattering experiments in beryllium and paraffin produced effects not explainable by known charged particles. The absence of charge was the central difficulty, as detection instruments of that era were mostly sensitive to charged radiation through deflection or ionisation patterns. Why Other Options Are Wrong: Because it is present only inside the nucleus: Protons are also in the nucleus and were still inferred earlier through other experiments, so this is not the main reason. Because it is a fundamental particle: Electrons and protons are also fundamental in many senses; this does not explain the delay. Because it does not move: Neutrons move and participate in nuclear reactions, so this statement is incorrect. Because it is heavier than the proton: Mass alone does not prevent discovery; detection difficulty arose mainly from the lack of electric charge, not from mass. Common Pitfalls: Students often think the neutron was discovered late simply because it is located deep inside the nucleus, forgetting that protons are also nuclear particles. The key idea is that experimental detection depends strongly on charge. Neutral particles require indirect detection techniques through collisions or secondary effects. Remember that when comparing discovery histories, the ability of a particle to interact with detectors, especially via electromagnetic forces, is crucial. Final Answer: The discovery of the neutron was delayed mainly because it does not carry any electric charge, making it much harder to detect with early experimental methods.

Question 5

Which one of the following combinations forms a homogeneous mixture: starch and sugar, methanol and water, graphite and charcoal, or calcium carbonate and calcium bicarbonate?

Accepted Answer

Introduction / Context: This question tests your understanding of the difference between homogeneous and heterogeneous mixtures. Identifying whether a given combination of substances forms a uniform mixture or not is a key skill in basic chemistry and in laboratory practice. Given Data / Assumptions: Several pairs of substances are listed: starch and sugar, methanol and water, graphite and charcoal, calcium carbonate and calcium bicarbonate and sand with iron filings. We are asked which pair forms a homogeneous mixture. Homogeneous mixture means uniform composition throughout and no visible boundaries between components. We assume room temperature and typical conditions unless stated otherwise. Concept / Approach: Homogeneous mixtures, such as true solutions, have components that are evenly distributed at the molecular or ionic level. Heterogeneous mixtures show visible phases or non uniform distribution. Liquids that are completely miscible, like methanol and water, form homogeneous mixtures. Solid solid combinations usually remain heterogeneous unless one solid dissolves into the other or forms an alloy, which is not the case here for the listed pairs. Step-by-Step Solution: 1) Starch and sugar are two distinct solids. When mixed as powders, they remain physically separate particles, so the mixture is heterogeneous.2) Methanol is an alcohol that is completely miscible with water in all proportions, forming a clear, uniform solution.3) Therefore, methanol and water together form a homogeneous mixture because no phase boundary is visible and composition is uniform.4) Graphite and charcoal are both forms of carbon, but mixing them simply gives a physical mixture of particles, which is heterogeneous.5) Calcium carbonate and calcium bicarbonate remain separate solids in a simple mixture, and sand with iron filings clearly shows two distinct solid phases, so these are heterogeneous as well. Verification / Alternative check: We can think in terms of phases. A homogeneous mixture should look like a single phase to the naked eye, such as a clear liquid or uniform solid solution. A methanol water mixture appears as a single, transparent liquid, confirming homogeneity. In contrast, mixed powders or granules almost always show visible specks of each component, indicating multiple phases. Thus, all the solid solid pairs in the options are heterogeneous, which further confirms methanol and water as the only correct choice. Why Other Options Are Wrong: Starch and sugar: Mixing these powders yields distinct particles, not a uniform single phase; this is a heterogeneous mixture. Graphite and charcoal: Even though both are forms of carbon, they do not dissolve into each other and remain separate solid particles. Calcium carbonate and calcium bicarbonate: These are different salts, and a simple physical mixture of their powders is heterogeneous. Sand and iron filings: Both components are clearly visible and can be separated with a magnet, showing a heterogeneous mixture. Common Pitfalls: Students sometimes think that any pair of substances made of similar elements, such as graphite and charcoal, must form a homogeneous mixture. However, similarity in composition does not guarantee homogeneity. Another mistake is to forget that miscible liquids like alcohol and water always form homogeneous solutions. A good habit is to ask whether the mixture shows a single uniform phase or multiple distinct phases to classify it correctly. Final Answer: The homogeneous mixture among the given pairs is formed by methanol and water.

Question 6

Which one of the following substances does not have a sharp and definite melting point?

Accepted Answer

Introduction / Context: This question examines your knowledge of the physical properties of crystalline versus amorphous solids. In particular, it asks which substance lacks a sharp melting point, a key diagnostic feature that distinguishes amorphous materials like glass from crystalline solids such as salts and metals. Given Data / Assumptions: Options include bromine, sodium chloride, mercury, glass and ice. We are asked to identify the substance that does not have a single, well defined melting point. We assume standard atmospheric pressure and ordinary laboratory conditions. Crystalline solids typically have sharp melting points, whereas amorphous solids soften over a range. Concept / Approach: Crystalline solids have long range order and a highly organised lattice structure, so they undergo a sharp phase transition at a specific temperature, called the melting point. Amorphous solids lack this long range order and gradually soften over a temperature range instead of melting sharply. Glass is a classic amorphous material and shows such behaviour. Liquids like bromine and mercury have well defined freezing points corresponding to their melting points, and ionic crystals like sodium chloride and molecular solids like ice also melt sharply at characteristic temperatures. Step-by-Step Solution: 1) Sodium chloride is an ionic crystalline solid with a high melting point around 801 degrees Celsius and melts sharply at that temperature.2) Ice is the crystalline form of water with a sharp melting point at 0 degrees Celsius under normal pressure.3) Metals and simple molecular substances such as mercury and bromine have specific freezing or melting points, even though some are liquids at room temperature.4) Glass, on the other hand, is an amorphous solid with disordered molecular arrangement and does not have a fixed melting point.5) When heated, glass gradually softens and flows over a range of temperatures instead of changing suddenly from solid to liquid at one specific value. Verification / Alternative check: In materials science, glass is often described as a supercooled liquid or amorphous solid. It exhibits a glass transition temperature region rather than a single melting point. When you heat glass, there is no sharp phase change; instead, viscosity decreases smoothly. Crystalline materials like sodium chloride and ice are used as calibration standards precisely because their melting points are well defined, which confirms that glass is the correct answer here. Why Other Options Are Wrong: Bromine: It is a liquid at room temperature but has a definite freezing and melting point when it turns to solid. Sodium chloride: A typical crystalline salt with a sharp and high melting point. Mercury: Although a liquid at ordinary conditions, it has a fixed freezing point around minus 39 degrees Celsius, equivalent to a sharp melting point. Ice: The solid form of water, which melts sharply at 0 degrees Celsius at one atmosphere pressure. Common Pitfalls: Students sometimes think that liquids like bromine and mercury have no melting points because they are already liquid. In reality, every pure substance has a well defined temperature at which it changes between solid and liquid under given pressure. Another confusion arises from the everyday idea that glass is a solid like a crystal, but in physics and chemistry it is classified as amorphous and does not have a sharp melting point. Remember that the key clue is the term amorphous versus crystalline. Final Answer: The substance that does not possess a sharp, definite melting point is glass.

Question 7

When sodium carbonate crystals lose their water of crystallisation on exposure to air, this behaviour is known by which term?

Accepted Answer

Introduction / Context: This question deals with the characteristic behaviour of hydrated salts such as sodium carbonate decahydrate when exposed to air. Knowing the terms efflorescence, deliquescence and hygroscopy is important in physical chemistry, especially in topics about hydration and interaction of solids with atmospheric moisture. Given Data / Assumptions: Sodium carbonate crystals, such as washing soda, are hydrated and contain water of crystallisation. On exposure to air, they lose some or all of this water. We are asked to name this property correctly. Options include hygroscopy, deliquescence, efflorescence, sublimation and hydrolysis. Concept / Approach: Efflorescence is the process by which a crystalline hydrated salt loses water of crystallisation when exposed to air, often due to the vapour pressure of the water in the crystal being higher than that of the surrounding atmosphere. Deliquescence is the opposite behaviour, where a substance absorbs so much moisture from the air that it dissolves in the absorbed water. Hygroscopic substances simply absorb moisture from the air without necessarily dissolving or losing water of crystallisation. Sublimation refers to solid to vapour transition, and hydrolysis refers to chemical reaction with water, so neither of these match the described behaviour. Step-by-Step Solution: 1) Sodium carbonate crystals, Na2CO3·10H2O, contain water molecules in their crystal lattice.2) When left exposed to dry air, these crystals gradually lose part or all of their water of crystallisation.3) As a result, the crystals may become powdery and less transparent.4) This loss of water of crystallisation to the atmosphere is termed efflorescence.5) Therefore, the property described in the question is efflorescence, not hygroscopy or deliquescence. Verification / Alternative check: Common textbook examples of efflorescent substances include sodium carbonate decahydrate and certain hydrated sodium sulphates. These salts visibly lose water and appear crusty when left in open air. Deliquescent substances, in contrast, include calcium chloride and magnesium chloride, which absorb moisture and become wet or liquefied. Sodium carbonate does not typically become liquid; instead, it dries out, confirming that efflorescence is the correct classification for its behaviour. Why Other Options Are Wrong: Hygroscopy: Hygroscopic substances mainly absorb moisture; they do not lose water of crystallisation to the surroundings as described here. Deliquescence: Deliquescent solids absorb so much water from air that they eventually dissolve in the absorbed water, which is opposite to the behaviour of sodium carbonate in the question. Sublimation: This refers to a solid changing directly to vapour, which is characteristic of substances like naphthalene, not hydrated salts losing water of crystallisation. Hydrolysis: This involves chemical reaction with water to form new products; simply losing water molecules from the crystal lattice is not hydrolysis. Common Pitfalls: Students often mix up the terms efflorescence and deliquescence because both involve interaction with atmospheric moisture. The key difference is the direction of water movement. In efflorescence, water leaves the crystal and moves to the air, whereas in deliquescence water from the air enters the solid, sometimes leading to a solution. Remembering a few standard examples like sodium carbonate for efflorescence and calcium chloride for deliquescence helps avoid confusion in exam questions. Final Answer: The loss of water of crystallisation from sodium carbonate crystals on exposure to air is called efflorescence.

Question 8

The absolute value of the charge on the electron was first determined in which classic atomic physics experiment?

Accepted Answer

Introduction / Context: This question belongs to the history of atomic structure and experimental physics. It asks which experiment measured the absolute charge of the electron, an essential constant in physics and chemistry known as the elementary charge. Given Data / Assumptions: Several famous experiments are listed: cathode ray, oil drop, alpha scattering, neutron discovery and canal ray experiments. We are specifically interested in the experiment that gave the numerical value of the electron charge. We assume basic familiarity with the roles of these classic experiments. The elementary charge is about 1.6 * 10^-19 coulomb. Concept / Approach: J. J. Thomson's cathode ray work determined the charge to mass ratio (e/m) of the electron, not its absolute charge. Rutherford's alpha scattering experiment gave information about the nucleus. Chadwick's work identified the neutron, and Goldstein's canal ray experiment led to the discovery of positive rays and protons. The direct measurement of the electron charge was achieved by Robert A. Millikan in his oil drop experiment by carefully balancing electric and gravitational forces on tiny charged oil droplets. Step-by-Step Solution: 1) Remember that the cathode ray experiment yielded the e/m ratio, not e alone.2) To find the absolute value of e, another independent measurement was required.3) Millikan's oil drop experiment involved spraying fine oil droplets into a chamber and subjecting them to an electric field.4) By adjusting the electric field so that the gravitational force on a charged droplet was balanced by electric force, Millikan could calculate the charge on individual droplets.5) He found that all measured charges were integer multiples of a smallest value, the elementary charge, thus determining the absolute charge of the electron. Verification / Alternative check: Science history clearly records that the numerical value of the elementary charge, approximately 1.6 * 10^-19 coulomb, comes from Millikan's oil drop experiment. Combining this value of e with Thomson's earlier value of e/m later allowed scientists to calculate the electron mass. No other experiment among the options directly measured the charge magnitude in this manner, which confirms that Millikan's experiment is the correct answer. Why Other Options Are Wrong: J. J. Thomson's cathode ray experiment: Established the existence of electrons and their e/m ratio but not the absolute value of e. Rutherford's alpha scattering experiment: Provided evidence for a small, dense atomic nucleus, not electron charge. Chadwick's neutron discovery experiment: Concerned neutral neutrons, not electron charge measurement. Goldstein's canal ray experiment: Led to the discovery of positive rays and protons but did not measure electron charge. Common Pitfalls: Students sometimes confuse the electron discovery experiment with the charge measurement experiment and wrongly choose Thomson. It helps to remember that the oil drop experiment is always associated with measuring the elementary charge, while cathode ray work is linked to discovering the electron and finding e/m. Keeping separate mental tags for what each historic experiment achieved reduces such confusion in multiple choice questions. Final Answer: The absolute value of the electron charge was first determined in R. A. Millikan's oil drop experiment.

Question 9

Which one of the following elements is essential as a structural material in nuclear reactors because of its low neutron absorption cross section?

Accepted Answer

Introduction / Context: This question belongs to applied chemistry and nuclear technology. It asks which element is widely used in the construction of nuclear reactor components because of its special nuclear properties, particularly its interaction with neutrons. Given Data / Assumptions: The elements listed are cobalt, nickel, zirconium, tungsten and chromium. We are looking for an element used as a structural material in nuclear reactors. An important requirement for such materials is low neutron absorption, especially for fuel cladding and other components close to the reactor core. We assume basic knowledge of common reactor materials. Concept / Approach: Inside a nuclear reactor, neutrons must be carefully controlled to sustain the chain reaction. Structural materials that surround the fuel rods should not absorb too many neutrons; otherwise, they would act as neutron poisons and reduce reactor efficiency. Zirconium and its alloys, especially zircaloy, are famous for their very low neutron absorption cross section and good corrosion resistance in high temperature water, making them ideal for fuel cladding. Other listed metals are used in industry but are not as suitable for this specific nuclear function. Step-by-Step Solution: 1) Cobalt and nickel are transition metals used in superalloys and magnet applications, but they have higher neutron absorption and are not standard for fuel cladding.2) Tungsten has a very high melting point and is used in filaments and electrodes but significantly absorbs neutrons and is dense.3) Chromium is commonly used in stainless steel for corrosion resistance but again is not the primary choice for low neutron absorbing cladding.4) Zirconium has excellent corrosion resistance in water and steam at high temperatures and, most importantly, a low neutron absorption cross section.5) Therefore, zirconium and its alloys are widely used as structural components, especially as cladding for fuel rods in many types of nuclear reactors. Verification / Alternative check: Standard descriptions of pressurised water reactors and boiling water reactors list zircaloy, a zirconium based alloy, as the common cladding material for uranium dioxide fuel pellets. This fact is directly linked to zirconium's nuclear properties. In contrast, cobalt, nickel, tungsten and chromium are rarely mentioned as primary reactor structural materials because of their less favourable neutron interaction characteristics and other material constraints. Why Other Options Are Wrong: Cobalt: Important in high strength alloys and magnetic materials, but its neutron absorption characteristics are not suitable for widespread use as reactor cladding. Nickel: Used in stainless steels and alloys but not preferred as a main low absorption cladding material. Tungsten: Though extremely refractory, it has high neutron absorption and is not commonly used as structural material inside the reactor core. Chromium: Primarily used in steel for corrosion resistance; it does not match zirconium in low neutron absorption performance for reactor applications. Common Pitfalls: Students sometimes choose metals like tungsten simply because they know it has a high melting point and seems robust. However, in nuclear reactors, neutron absorption characteristics are as important as mechanical strength. Another mistake is to focus on general industrial use rather than the specialised requirements of reactor design. Remember that zirconium's key advantage is its combination of corrosion resistance and low neutron capture, which is exactly what this question is testing. Final Answer: The element widely used as a structural material in nuclear reactors is zirconium.

Question 10

An atom has 7 electrons in its M shell and contains 18 neutrons in its nucleus. What is the mass number of this atom?

Accepted Answer

Introduction / Context: This atomic structure question combines ideas about electronic configuration and nuclear composition. It asks you to interpret the number of electrons in a particular shell, identify the element and then calculate its mass number using the given number of neutrons. Given Data / Assumptions: The atom has 7 electrons in its M shell (the third shell, n = 3). The nucleus contains 18 neutrons. The atom is assumed to be neutral unless stated otherwise, so number of protons equals number of electrons. We must find the mass number, which equals the sum of protons and neutrons. Concept / Approach: Electron shells are filled in the order K (n = 1), L (n = 2), M (n = 3) and so on according to well known rules and the capacity of each shell. The M shell can hold up to 18 electrons but may hold fewer depending on the element. If an atom has 7 electrons in the M shell, we can deduce its total number of electrons by considering typical configurations such as 2, 8, 7. Once we know the atomic number, which equals the number of protons, we can add the given neutrons to obtain the mass number A = Z + N. Step-by-Step Solution: 1) For lighter elements with electrons reaching the M shell, a common pattern is K = 2 electrons, L = 8 electrons, M = remaining electrons.2) If the M shell has 7 electrons, a natural configuration is 2, 8, 7, giving a total of 2 + 8 + 7 = 17 electrons.3) In a neutral atom, number of electrons equals number of protons, so the atomic number Z = 17.4) The problem states that the nucleus contains 18 neutrons, so N = 18.5) Mass number A is defined as A = Z + N = 17 + 18 = 35. Verification / Alternative check: An element with atomic number 17 is chlorine. A common isotope of chlorine has mass number 35, written as Cl 35, with 17 protons and 18 neutrons. This matches the numbers we computed. The electron configuration of chlorine is indeed 2, 8, 7, which confirms that the assumption about distribution among shells is correct and that the calculated mass number of 35 is consistent with known isotopes. Why Other Options Are Wrong: 25: This would imply 8 neutrons if Z = 17, which does not match the given 18 neutrons. 27: Again inconsistent, as 27 minus 17 equals 10 neutrons, not the 18 specified. 43: This would require 26 neutrons in addition to 17 protons, which does not match the question data. 37: This would imply 20 neutrons, whereas the nucleus is stated to contain 18 neutrons. Common Pitfalls: Students sometimes miscount the total electrons by assuming the M shell is full when they see it mentioned. It is important to use the actual number given, 7 in this case, and not the maximum possible. Another common error is to forget that mass number includes both protons and neutrons and mistakenly use only the number of neutrons. Always remember that A = Z + N and that Z is determined from the total electrons for a neutral atom. Final Answer: The mass number of the atom described is 35.

Question 11

In a sodium ion Na+, how many electrons are present around the nucleus?

Accepted Answer

Introduction / Context: This basic chemistry question checks your understanding of how the number of electrons changes when a neutral atom forms a cation. It uses the sodium ion Na+ as a common example from ionic bonding and periodic table topics. Given Data / Assumptions: We are dealing with the sodium ion, symbol Na+. Atomic number of sodium (Na) is 11. The ion has a single positive charge. We assume the ion is formed by loss of electrons, not by adding protons. Concept / Approach: The atomic number of an element equals the number of protons in its nucleus and also equals the number of electrons in the neutral atom. When a positive ion (cation) is formed, the atom loses one or more electrons. The charge on the ion tells you how many electrons have been lost or gained. For Na+, the plus one charge indicates that one electron has been lost relative to the neutral sodium atom. The number of protons does not change in this process. Step-by-Step Solution: 1) Neutral sodium atom has atomic number 11, so it possesses 11 protons and 11 electrons.2) The ion Na+ has a charge of plus one.3) A plus one charge means the ion has one fewer electron than the neutral atom, because electrons carry negative charge.4) Therefore, electrons in Na+ = 11 (in neutral atom) minus 1 (lost) = 10 electrons.5) The number of protons remains 11, but the loss of one electron leads to a stable configuration similar to neon. Verification / Alternative check: We can cross check using the electron configuration. Neutral sodium has configuration 2, 8, 1. When it loses one electron to form Na+, the configuration becomes 2, 8, which totals 10 electrons. This is the same configuration as neon, a noble gas, explaining the stability of Na+ and confirming that the correct electron count is 10 in the sodium ion. Why Other Options Are Wrong: 1: This would correspond to a hydrogen like ion and is far too low for sodium. 11: This is the number of electrons in neutral sodium, not in Na+ which has lost one electron. 12: This would imply an extra electron and describe Na- rather than Na+, which is not the ion in the question. 9: This would correspond to losing two electrons from sodium, giving a Na2+ ion, which is not common and not what is specified. Common Pitfalls: A common mistake is to confuse the number of electrons with the atomic number even for ions, forgetting that electrons can be lost or gained. Another error is to think that a positive charge indicates extra protons rather than fewer electrons. In reality, nuclear protons remain fixed in chemical changes; only electrons are transferred. Always adjust the electron count according to the ionic charge, keeping the proton count equal to the atomic number. Final Answer: The sodium ion Na+ contains 10 electrons around its nucleus.

Question 12

'Misch metal' is an alloy widely used in the manufacture of which of the following everyday products?

Accepted Answer

Introduction / Context: This general science question asks about a specific alloy called misch metal and its common use in daily life. Knowing typical applications of alloys helps in understanding material selection in technology and industry. Given Data / Assumptions: The alloy in question is misch metal. Options list various products such as car brakes, smoke detectors, cigarette lighters, emergency lights and transformers. We are asked to identify the product for which misch metal is widely used. We assume standard knowledge from textbooks on alloys and their uses. Concept / Approach: Misch metal is an alloy of rare earth metals, primarily cerium and lanthanum, along with small amounts of other elements. It has the special property that it produces sparks when struck against a rough surface. This makes it highly suitable for use in lighter flints. Therefore, misch metal is widely used in the manufacture of the flint portion of cigarette lighters, where controlled sparking is required to ignite fuel. Step-by-Step Solution: 1) Identify misch metal as a mixture of rare earth elements, rich in cerium.2) Recall that one of its key properties is the ability to produce a shower of hot sparks when scraped or struck.3) Consider the listed products and ask which one needs a reliable spark source as a core function.4) Cigarette lighters have a small flint that is rubbed by a rotating wheel to create sparks to ignite the fuel.5) Therefore, misch metal is most closely associated with the manufacture of flints in cigarette lighters. Verification / Alternative check: Standard references on alloys mention misch metal as the main component of lighter flints, often in combination with iron to make ferrocerium. When you operate a traditional lighter, the sparks you see are produced by abrasion of this misch metal based alloy. None of the other listed applications rely primarily on such sparking properties, which confirms that cigarette lighters are the intended answer. Why Other Options Are Wrong: Material for car brakes: Brake pads and discs are generally made from steel, cast iron or specialised friction materials, not misch metal. Smoke detectors: These typically use small amounts of radioactive material or electronic sensors, and do not require misch metal. Emergency lights: These depend on batteries and light sources such as LEDs or fluorescent lamps, not on spark producing alloys. Electrical transformers: Transformers are built from copper windings and laminated iron or silicon steel cores, not misch metal. Common Pitfalls: Some students may wrongly associate misch metal with high strength structural applications because the name sounds technical. Others might confuse it with alloys used in car components. The key to remembering this fact is to focus on its sparking property. Any time you see a question about misch metal, think of lighter flints and sparks, which will guide you to the correct application in exams. Final Answer: 'Misch metal' is widely used in the manufacture of flints for cigarette lighters.

Question 13

Which one of the following metallic elements is known to be the poorest conductor of heat among the choices given?

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Introduction / Context: This question comes from basic physics and chemistry of metals. It asks you to identify which metal among the listed options is the poorest conductor of heat, a property that influences how materials are used in cookware, wiring and thermal insulation. Given Data / Assumptions: The options are the metals sodium, lead, zinc, mercury and aluminium. We are comparing thermal conductivity, not electrical conductivity. We assume typical textbook comparisons among common metals. We are interested in the relatively poorest conductor of heat among the options given. Concept / Approach: Metals in general are good conductors of heat due to the presence of free electrons and closely packed atoms. However, different metals have different thermal conductivities. Light metals like copper and aluminium conduct heat very well, while some heavy metals like lead conduct heat relatively poorly compared with other common metals. In many basic exam questions, lead is cited as the poorest conductor of heat among typical structural metals. Step-by-Step Solution: 1) Aluminium is widely used in utensils and heat sinks because it is a very good conductor of heat, so it cannot be the poorest.2) Sodium is a highly reactive alkali metal and, as a metal, has good thermal conductivity, though it is not usually used structurally.3) Zinc also conducts heat reasonably well and is used in alloys and galvanisation rather than as a thermal insulator.4) Mercury is a liquid metal at room temperature and has decent thermal conductivity, but in many secondary level comparisons lead is highlighted as the poorest heat conducting metal among common solids.5) Therefore, lead is regarded as the poorest conductor of heat among the given metallic options in typical exam contexts. Verification / Alternative check: Standard data tables in many textbooks and reference books list approximate thermal conductivities and note that lead has much lower thermal conductivity than metals like copper, aluminium and zinc. This is why lead is sometimes used where heat conduction needs to be reduced, such as in some shielding or protective applications. Although mercury is a liquid and has its own special properties, school level questions traditionally identify lead as the answer to this comparison. Why Other Options Are Wrong: Sodium: Being an alkali metal with free electrons, it conducts heat well compared to lead. Zinc: It has better thermal conductivity and is not usually cited as the poorest conductor among metals. Mercury: While a metal, it is not typically given as the standard example in such questions; lead is more commonly identified as the poorer solid metal conductor. Aluminium: Known for high thermal conductivity and used for cooking utensils and heat exchangers, so it is clearly not the poorest. Common Pitfalls: Students sometimes overthink this question by focusing on the fact that mercury is a liquid and guessing it might conduct heat poorly. However, thermal conductivity depends on atomic structure and free electrons, not simply on physical state. For most standard competitive exams, the expected answer is lead. Remembering that copper and aluminium are excellent conductors and that lead is the typical poor conductor among metals helps avoid confusion. Final Answer: The poorest conductor of heat among the given metallic elements is lead.

Question 14

When liquid water freezes to form ice, how does its density change?

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Introduction / Context: This question addresses a very important and unusual physical property of water. Unlike most substances, water expands on freezing, and this has direct consequences for density, floating of ice and many environmental phenomena. Given Data / Assumptions: We consider pure water at normal atmospheric pressure. The change is from liquid water to solid ice. We are asked what happens to density during this phase change. Density is defined as mass per unit volume. Concept / Approach: Most substances contract on cooling and thus increase in density when they solidify. Water behaves unusually because of its hydrogen bonding. As water freezes, molecules arrange themselves in an open hexagonal lattice that occupies more volume than the liquid. Since mass remains the same while volume increases, density decreases. This lower density of ice compared with liquid water causes ice to float on water, an effect with major environmental significance. Step-by-Step Solution: 1) Density is given by density = mass / volume.2) When water at around 4 degrees Celsius cools and freezes, its molecules arrange into a rigid structure with more open space.3) The mass of the sample is unchanged during freezing.4) However, the volume increases due to the open lattice structure of ice.5) With a larger volume and the same mass, the density decreases, which means ice is less dense than liquid water. Verification / Alternative check: Numerical values from data tables show that the density of water at around 4 degrees Celsius is close to 1.0 gram per cubic centimetre, while the density of ice at 0 degrees Celsius is about 0.917 gram per cubic centimetre. This confirms that the density is lower in the solid state. The everyday observation that ice cubes float on water is direct evidence of this density difference, since floating occurs when the solid is less dense than the surrounding liquid. Why Other Options Are Wrong: It remains constant: If density were constant, ice would neither sink nor float and there would be no expansion on freezing, which is contrary to observation. It increases: This is what happens for most substances but not for water; if density increased, ice would sink instead of floating. It becomes zero: Density can never become zero for a real substance with non zero mass and finite volume. It fluctuates randomly: The density change is systematic and predictable, not random. Common Pitfalls: Some students assume that all substances become denser when they solidify because this is generally true. Water is a famous exception due to hydrogen bonding and its special molecular structure. Forgetting this anomaly leads to wrong answers. Remember the key visual clue: ice floats on water, so its density must be lower than that of liquid water. This simple observation should guide you to the correct choice in exam questions. Final Answer: When water freezes to form ice, its density decreases.

Question 15

How many moles of silicon are present in a 140 g sample, given that the atomic mass of silicon is 28 g per mole?

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Introduction / Context: This numerical chemistry question tests the basic concept of the mole, which is central to stoichiometry and quantitative chemical calculations. It asks you to convert a given mass of an element into moles using its atomic mass. Given Data / Assumptions: Mass of silicon sample = 140 g. Atomic mass of silicon = 28 g per mole. We assume pure silicon and standard atomic mass for calculations. We have to calculate the number of moles of silicon in the sample. Concept / Approach: The mole concept states that one mole of a substance contains a fixed number of particles (Avogadro number) and has a mass in grams equal to its molar or atomic mass. The relationship between mass, molar mass and moles is simple: moles = given mass / molar mass. By inserting the given mass of silicon and its molar mass into this formula, we can directly compute the number of moles in the sample. Step-by-Step Solution: 1) Write the formula: number of moles = given mass (g) / molar mass (g per mole).2) For silicon, molar mass = 28 g per mole.3) Given mass of silicon sample = 140 g.4) Substitute into the formula: moles of silicon = 140 / 28.5) Perform the division: 140 / 28 = 5, so there are 5 moles of silicon in the sample. Verification / Alternative check: We can check the arithmetic by noting that 28 * 5 = 140, which confirms that 5 moles of silicon have a mass of 140 g. We could also set up a simple proportion: 28 g corresponds to 1 mole, so 140 g corresponds to x moles. Then x = 140 * 1 / 28 = 5 moles, which matches the earlier calculation. Why Other Options Are Wrong: 10: This would correspond to a mass of 10 * 28 = 280 g, not the given 140 g. 28: This represents the atomic mass in grams per mole, not the number of moles for a 140 g sample. 140: This is the mass in grams, not the number of moles; confusing mass with moles is a common error. 4: This would correspond to a mass of 4 * 28 = 112 g, which is less than the given 140 g. Common Pitfalls: Students sometimes mix up the roles of molar mass and moles, treating the atomic mass number itself as the answer. Others may forget to divide the given mass by molar mass and instead multiply. To avoid these mistakes, always remember the simple relationship moles = mass / molar mass and double check units to ensure grams cancel correctly, leaving moles as the final unit. Final Answer: The 140 g sample of silicon contains 5 moles of silicon atoms.

Question 16

In the air that we breathe, which gas is the third most abundant component after nitrogen and oxygen?

Accepted Answer

Introduction / Context: This general science question is about the composition of the Earth atmosphere. It asks which gas ranks third in abundance after the two main components, nitrogen and oxygen, in dry air at sea level. Given Data / Assumptions: We are considering dry, unpolluted air at ground level. Nitrogen and oxygen are known to be the first and second most abundant gases. The options include argon, neon, carbon dioxide, hydrogen and ozone. We use approximate percentage compositions commonly quoted in textbooks. Concept / Approach: Dry air is mainly composed of nitrogen (about 78 percent) and oxygen (about 21 percent). The third most abundant gas by volume is argon, an inert noble gas, present at about 0.93 percent. Carbon dioxide accounts for only about 0.04 percent, and other gases like neon, hydrogen and ozone are present in much smaller trace amounts. Therefore, among the options, argon clearly ranks third in abundance. Step-by-Step Solution: 1) Recall the approximate composition of dry air: nitrogen around 78 percent, oxygen around 21 percent.2) Together, nitrogen and oxygen make up roughly 99 percent of the atmosphere.3) The next largest fraction is argon, at about 0.93 percent by volume.4) Carbon dioxide is around 0.04 percent on average, much less than argon.5) Neon, hydrogen and ozone occur in even smaller trace amounts, so they cannot be the third most abundant gas. Verification / Alternative check: Standard environmental science and chemistry references list dry air composition as approximately 78 percent nitrogen, 21 percent oxygen, 0.93 percent argon and 0.04 percent carbon dioxide, with all other gases together making up less than 0.1 percent. This clearly confirms that argon is the third most abundant gas after nitrogen and oxygen under normal conditions. Why Other Options Are Wrong: Neon: Present only in tiny trace amounts, far below one percent, so it is not the third most abundant gas. Carbon dioxide: Despite its importance for climate and photosynthesis, it makes up only about 0.04 percent of air, less than argon. Hydrogen: Exists in very small trace concentrations in the atmosphere and is not a major component of air. Ozone: Mostly concentrated in the upper atmosphere and present in extremely low concentrations near the surface, not significant in total volume. Common Pitfalls: Students sometimes choose carbon dioxide because it is frequently mentioned in discussions about global warming and the greenhouse effect. However, importance for climate does not mean abundance by volume. Always distinguish between relative concentration and environmental impact. Remember the simple order for air composition questions: nitrogen first, oxygen second and argon third. Final Answer: The third most abundant gas in the air we breathe, after nitrogen and oxygen, is argon.

Question 17

Which metal among calcium, sodium, magnesium and potassium reacts most violently with cold water?

Accepted Answer

Introduction / Context: This question tests your knowledge of metal reactivity, particularly the reaction of alkali and alkaline earth metals with cold water. Such reactions are important in safety considerations and in understanding periodic trends in chemical reactivity. Given Data / Assumptions: The metals listed are calcium, sodium, magnesium, potassium and aluminium. We are considering their reaction with cold water. We must identify which metal reacts most vigorously. We assume pure metals and typical laboratory or textbook conditions. Concept / Approach: In the periodic table, reactivity of alkali metals (Group 1) increases as we move down the group. Lithium is least reactive, followed by sodium, then potassium, rubidium and caesium. Potassium reacts more vigorously with cold water than sodium. Alkaline earth metals (Group 2) like magnesium and calcium are generally less reactive with cold water than alkali metals. Magnesium reacts very slowly with cold water, whereas calcium reacts moderately, forming calcium hydroxide and hydrogen gas. Step-by-Step Solution: 1) Sodium and potassium are alkali metals and known to react strongly with cold water, producing hydrogen gas and metal hydroxide.2) Potassium is lower in Group 1 than sodium, and its outer electron is more easily lost, making it more reactive.3) Magnesium is an alkaline earth metal and reacts only very slowly with cold water; its reaction is much less vigorous.4) Calcium reacts with cold water more than magnesium but still less violently than sodium and potassium.5) Therefore, among the given metals, potassium shows the most violent reaction with cold water. Verification / Alternative check: In standard school demonstrations, small pieces of sodium and potassium are placed in water. Sodium moves rapidly on the surface and may ignite hydrogen, while potassium often reacts so vigorously that the hydrogen catches fire with a lilac flame almost immediately. Magnesium is usually demonstrated with steam rather than cold water because its reaction with cold water is very slow. Calcium, although more reactive than magnesium with water, does not show as explosive a reaction as potassium. These observations confirm that potassium is the most reactive with cold water among the listed metals. Why Other Options Are Wrong: Calcium: Reacts with cold water but not as violently as sodium and potassium; the reaction is more gentle and produces milky calcium hydroxide. Sodium: Very reactive with water but still less reactive than potassium in the same conditions. Magnesium: Reacts only slightly with cold water; the reaction is slow and not violent. Aluminium: Forms a protective oxide layer and does not react vigorously with cold water under normal conditions. Common Pitfalls: Students sometimes choose sodium because it is commonly used in textbook examples of vigorous metal water reactions. However, periodic trends show that reactivity increases down Group 1, making potassium more reactive than sodium. Another error is to ignore group trends and pick calcium or magnesium due to familiarity. To answer such questions correctly, always recall the order of reactivity within a group and remember that Group 1 metals are generally more reactive with water than Group 2 metals. Final Answer: The metal that reacts most violently with cold water among the options is potassium.

Question 18

In basic chemistry, which chemical elements are present in the organic compound urea (CO(NH2)2)?

Accepted Answer

Introduction / Context: Urea is one of the most important organic compounds in basic chemistry and biochemistry. It is widely used as a nitrogen rich fertilizer in agriculture and is also an important metabolic waste product in many living organisms. To answer questions about its composition, you must be familiar with its molecular formula and understand which chemical elements are present in that formula. This type of question checks your understanding of chemical notation and elemental composition of a well known compound. Given Data / Assumptions: The compound under discussion is urea. The molecular formula of urea is CO(NH2)2. We assume standard chemical notation where C is carbon, H is hydrogen, O is oxygen and N is nitrogen. The task is simply to identify which elements appear in the formula, not how many atoms of each are present. Concept / Approach: The molecular formula of a compound lists the symbols of all elements present in a single molecule. Subscripts show the number of atoms of each element, but for this question we only need to know which symbols appear at all. Urea is written as CO(NH2)2. By carefully reading the formula from left to right, you can list each distinct element symbol. Once you identify carbon, oxygen, nitrogen and hydrogen in the formula, you match that list with the options given in the question stem. Step-by-Step Solution: Step 1: Write the molecular formula of urea clearly: CO(NH2)2.Step 2: Identify the first element symbol C, which stands for carbon.Step 3: Identify the next element symbol O, which stands for oxygen.Step 4: Inside the parentheses, note the symbol N for nitrogen.Step 5: Also inside the parentheses, note the symbol H for hydrogen.Step 6: Collect the distinct elements: C, O, N and H. This matches the option that lists all four of these elements together. Verification / Alternative check: An alternative way to check is to break urea into its functional parts. The central portion resembles a carbonyl group (C=O), which clearly contains carbon and oxygen. On each side of the carbonyl carbon there is an NH2 group, known as an amino group, which contains nitrogen and hydrogen. When you join these parts into the full molecule CO(NH2)2, both amino groups and the carbonyl group are still present. Therefore, all four elements carbon, oxygen, nitrogen and hydrogen must be part of the molecule. This confirms the list of elements we identified earlier. Why Other Options Are Wrong: Option a, C, H, O, omits nitrogen even though NH2 groups are clearly present. Option b, C, N, O, leaves out hydrogen, which is essential in the NH2 groups. Option c, C, N, H, incorrectly leaves out oxygen, even though there is a carbonyl oxygen in the CO part of the formula. Each of these options misses at least one element from the correct set, so they cannot represent the full elemental composition of urea. Common Pitfalls: A frequent mistake is to overlook elements that appear only once or to misread the formula by focusing only on the central CO portion and forgetting the attached NH2 groups. Students may also confuse the number of atoms with the number of elements and overthink the problem. To avoid these issues, always scan the entire formula and list each distinct element symbol at least once. Remember that subscripts, parentheses and coefficients affect atom counts but not the identity of the elements themselves. Final Answer: Urea contains the four elements carbon, oxygen, nitrogen and hydrogen (C, O, N, H) in its molecular formula CO(NH2)2.

Question 19

In the modern periodic table of known chemical elements, which category contains the maximum number of elements?

Accepted Answer

Introduction / Context: The periodic table is a systematic arrangement of all known chemical elements. These elements can be broadly grouped into metals, non metals and metalloids based on their physical and chemical properties. Many basic chemistry questions test whether you know which of these groups is the largest. This helps build intuition about why metallic properties are so common in nature and in industrial applications, ranging from structural materials to electrical conductors. Given Data / Assumptions: We are considering all known elements in the periodic table. The main categories under comparison are metals, non metals and metalloids. We assume standard school level classification into these three broad groups. The question asks which group has the largest count of elements, not which group is most abundant in the Earth's crust by mass. Concept / Approach: Most elements in the periodic table show metallic character, especially those towards the left and bottom of the table. Metals include alkali metals, alkaline earth metals, transition metals, lanthanides and actinides, as well as many post transition metals. Non metals are relatively few and located mainly on the upper right side. Metalloids form a narrow diagonal band with only a handful of elements. Therefore, when you compare the counts, metals clearly outnumber both non metals and metalloids by a wide margin. Step-by-Step Solution: Step 1: Visualise the periodic table and recall that most of the left and central regions are occupied by metallic elements.Step 2: Identify that the transition block (d block), lanthanides and actinides are all metals, providing a large number of metallic elements.Step 3: Note that non metals are concentrated mainly in groups such as 14, 15, 16 and 17 near the top right, plus hydrogen, but they are comparatively few.Step 4: Metalloids such as boron, silicon and arsenic form a small step like boundary between metals and non metals, numbering only a few elements.Step 5: Adding these observations, you see that metals form the majority of the known elements.Step 6: Therefore, the correct answer is that metals account for the maximum number of known elements. Verification / Alternative check: A practical way to check this is to recall how many element names you commonly see ending in typical metal suffixes such as "ium", for example sodium, calcium, titanium and uranium. These are all metals. In contrast, there are far fewer commonly known non metals and metalloids. Additionally, chemistry textbooks and reference charts usually state that over three quarters of the elements in the periodic table are metals. This repeated statement reinforces the conclusion that metals dominate in terms of count. Why Other Options Are Wrong: Option b, non metals, includes important elements like oxygen and nitrogen, but their total number is relatively small compared to the combined families of metals. Option c, metalloids, accounts for only a small group of elements with intermediate properties, such as silicon and germanium. Option d, none of these, is incorrect because one of the named groups clearly does have the maximum membership, and that group is metals. Therefore, these alternative options do not match the well known distribution of elements in the periodic table. Common Pitfalls: Students sometimes confuse abundance in nature with count in the periodic table. Non metals like oxygen and silicon containing compounds may be very abundant on Earth, but that does not mean they outnumber metals in the table. Another mistake is to misclassify certain elements, for example thinking that all elements near the metalloid line are non metals, which can distort your mental count. To avoid these issues, focus on the broad picture: the periodic table is visually dominated by metallic elements, especially in the central and lower regions. Final Answer: In the modern periodic table, the category that contains the maximum number of known elements is metals.

Question 20

A neutral nitrogen atom has 7 protons and 7 electrons. After gaining three electrons to form a nitride ion (N3-), how many protons and electrons does this nitride ion have?

Accepted Answer

Introduction / Context: Understanding the difference between atoms and ions is a fundamental concept in introductory chemistry. This question focuses on how the numbers of protons and electrons change when a neutral atom gains or loses electrons to form an ion. Nitrogen is a common example in questions about anions because it tends to gain three electrons to complete its valence shell and form the nitride ion, written as N3-. Recognising that proton number does not change during ion formation is a key idea being tested here. Given Data / Assumptions: A neutral nitrogen atom has 7 protons and 7 electrons. The nitride ion is represented as N3-. An anion forms when an atom gains electrons. We assume that forming N3- does not change the nucleus, so the number of protons stays constant. Concept / Approach: The atomic number of nitrogen is 7, which means that every nitrogen atom has 7 protons in its nucleus. The number of protons defines the identity of the element and does not change when ions are formed. A neutral atom has an equal number of protons and electrons, so neutral nitrogen has 7 electrons. When nitrogen forms N3-, it gains three extra electrons, giving it a total of 10 electrons. Thus, the correct description of the nitride ion must show 7 protons and 10 electrons. Any answer that changes the number of protons is automatically incorrect. Step-by-Step Solution: Step 1: Note that the atomic number of nitrogen is 7, so a nitrogen nucleus always contains 7 protons.Step 2: A neutral nitrogen atom has 7 electrons to balance the 7 positive charges of the protons.Step 3: The nitride ion N3- has a 3 negative charge, which indicates that the atom has gained three extra electrons.Step 4: Add the three extra electrons to the original 7 electrons: 7 + 3 = 10 electrons.Step 5: The number of protons remains 7 because the nucleus is not altered when forming a simple ion.Step 6: Therefore, the nitride ion has 7 protons and 10 electrons. Verification / Alternative check: You can verify the charge by subtracting electron count from proton count. For N3-, the net charge should be -3. With 7 protons and 10 electrons, the total positive charge is +7 and the total negative charge is -10. The overall charge is +7 + (-10) = -3, which matches the notation N3-. If you tried any other combination of protons and electrons from the options, you would not obtain the correct -3 charge, which confirms that 7 protons and 10 electrons is the only consistent pair. Why Other Options Are Wrong: Option a, 10 protons and 7 electrons, wrongly changes the number of protons and also leads to a net charge of +3 instead of -3. Option c, 4 protons and 7 electrons, changes the element entirely, because an atom with 4 protons would be beryllium, not nitrogen. Option d, 4 protons and 10 electrons, again represents a different element and a charge of -6. None of these alternatives keep the atomic number 7 and simultaneously achieve the correct -3 charge. Common Pitfalls: A common mistake is to assume that both protons and electrons change when ions form. In reality, only the electron count changes in simple ionic processes, while the nucleus (and thus the number of protons) remains the same. Another pitfall is misinterpreting the 3- charge as meaning the ion has only three electrons, which is incorrect. Instead, the charge indicates a difference of three between the numbers of protons and electrons. Always start with the atomic number to fix the proton count, then adjust the electron count according to the charge. Final Answer: The nitride ion N3- contains 7 protons and 10 electrons.

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