In atomic structure, which of the following conditions will cause an atom or species to be a negative ion (anion)?

Introduction / Context:
Ions are charged species formed when atoms gain or lose electrons. A basic distinction is between positive ions (cations) and negative ions (anions). This question asks which relationship between protons and electrons leads to the formation of a negative ion. Understanding this relationship helps in predicting charges on ions and balancing ionic equations.

Given Data / Assumptions:

• Protons carry a positive charge.
• Electrons carry an equal magnitude of negative charge.
• Neutrons are electrically neutral.
• The net charge on a species depends on the difference between the numbers of protons and electrons.

Concept / Approach:
In a neutral atom, the number of protons equals the number of electrons, so total positive and negative charges cancel. If an atom gains extra electrons, the negative charges outnumber the positive charges and the net charge becomes negative, creating an anion. If an atom loses electrons, the positive charges dominate and a cation is formed. The number of neutrons does not affect charge because neutrons are neutral. Therefore, the condition for a negative ion is that electrons must exceed protons in number.

Step-by-Step Solution:
Step 1: Recall that protons have charge plus one, electrons have charge minus one, and neutrons have charge zero in atomic units. Step 2: For a species to have a net negative charge, the total negative electron charge must be greater than the total positive proton charge. Step 3: This happens when the number of electrons is greater than the number of protons. Step 4: Recognise that relationships involving neutrons do not change charge because neutrons are neutral. Step 5: Choose the option that states the species has more electrons than protons as the condition for a negative ion.

Verification / Alternative check:
Take the chloride ion, Cl−, as an example. A neutral chlorine atom has 17 protons and 17 electrons. When it gains one electron, it has 17 protons and 18 electrons. The net charge is now minus one because the negative charges exceed the positive charges by one. This matches the rule more electrons than protons. Similarly, oxide ion O2− has 8 protons and 10 electrons, again more electrons than protons. These examples confirm that anions always have electron counts greater than proton counts.

Why Other Options Are Wrong:
- If it has more electrons than neutrons: Neutron count does not determine charge. A species can have any combination of electrons and neutrons and still be neutral or positive depending on proton count.
- If it has more protons than electrons: This leads to a net positive charge and forms a cation rather than a negative ion.
- If it has more protons than neutrons: This concerns nuclear composition and stability but has nothing to do with net electrical charge.
- If it has equal numbers of protons and electrons: That is the definition of a neutral atom with no net charge, not a negative ion.

Common Pitfalls:
Students sometimes focus too much on the neutron number because it changes in isotopes and nuclear processes. However, for charge, only the balance between protons and electrons matters. Another pitfall is mixing up cations and anions, especially with the sign of the charges. A simple memory trick is that anion starts with the letter A, like additional electrons, and cation with C, like fewer electrons and a positive centre. The key test remains whether electrons exceed protons for negative ions.

Final Answer:
An atom or species will be a negative ion if It has more electrons than protons.