In the context of melting points, which of the following elements has the lowest melting point under standard atmospheric pressure?

Introduction / Context:
Melting point is a fundamental physical property of elements and compounds, indicating the temperature at which a solid turns into a liquid under standard pressure. Metals and covalent solids often have high melting points, while noble gases and some molecular substances have very low melting points because their particles are held together by weak forces. This question asks you to identify which of the listed elements has the lowest melting point, contrasting a noble gas with several high melting point metals and nonmetals.

Given Data / Assumptions:

• The elements listed are platinum, carbon, cobalt, and krypton.
• We consider their melting points at standard atmospheric pressure.
• Platinum and cobalt are transition metals, carbon forms covalent networks, and krypton is a noble gas.
• We use general trends rather than exact numerical values for this multiple choice question.

Concept / Approach:
Metals like platinum and cobalt have strong metallic bonds and therefore high melting points. Carbon, in forms such as diamond, has extremely strong covalent network bonding and one of the highest melting points known. In contrast, krypton is a noble gas whose atoms are held together only by weak dispersion forces in the solid state. As a result, krypton condenses and freezes at very low temperatures, with a melting point far below that of metals and covalent solids. Thus, among the elements listed, krypton has by far the lowest melting point.

Step-by-Step Solution:
Step 1: Identify platinum as a dense transition metal used in high temperature applications, indicating a high melting point. Step 2: Recognise cobalt as another transition metal with strong metallic bonding and a high melting point. Step 3: Recall that carbon in its covalent network forms, such as diamond, has extremely strong bonds and one of the highest melting points among elements. Step 4: Note that krypton is a noble gas, existing as individual atoms with only weak intermolecular attractions at low temperatures. Step 5: Understand that noble gases solidify only at very low temperatures, so their melting points are much lower than those of metals and covalent solids. Step 6: Conclude that krypton has the lowest melting point among the options and select it as the correct answer.

Verification / Alternative check:
Reference data show that krypton has a melting point far below zero degrees Celsius, since it remains a gas at ordinary laboratory temperatures. Platinum and cobalt, by contrast, melt at temperatures of well over a thousand degrees Celsius, reflecting strong metallic bonding. Carbon in the form of diamond does not melt easily under normal pressure and is often cited as having one of the highest melting points of all substances. These large differences in melting point values confirm that krypton, as a noble gas, has the lowest melting point among the choices provided.

Why Other Options Are Wrong:
Platinum is renowned for its high melting point and is used in laboratory crucibles and other high temperature equipment; it clearly does not have a low melting point. Cobalt is also a high melting point metal, used in alloys that must withstand heat. Carbon in its strong covalent network forms has a melting point even higher than most metals. None of these elements can compete with the extremely low melting point of krypton, which solidifies only under very cold conditions and therefore is the correct choice for lowest melting point.

Common Pitfalls:
Students may be tempted to choose carbon or platinum simply because they appear more unusual or familiar from laboratory discussions, without recalling that these have very high melting points. Another pitfall is to overlook the physical state of krypton at room temperature; as a gas, it must have a very low melting point compared with solids. To avoid such mistakes, remember that noble gases are gases at ordinary temperatures and solidify only at very low temperatures, giving them extremely low melting points. Metals and covalent network solids generally have much higher melting points.

Final Answer:
Among the elements listed, the lowest melting point is for krypton, a noble gas with a very low melting point.