Which is the correct order of melting points of these Period 3 elements? A - AQA - A-Level Chemistry - Question 6 - 2017 - Paper 3

To determine the correct order of melting points, we must consider the types of bonding and structure in the Period 3 elements (phosphorus, sulfur, chlorine, and argon).

1. Argon: Being a noble gas, argon has a very low melting point due to weak van der Waals forces.
2. Chlorine: Chlorine is a diatomic molecule held together by van der Waals forces, which are stronger than those in argon but still relatively weak.
3. Sulfur: Sulfur consists of S8 molecules, which have a higher melting point due to the increased number of atoms, leading to stronger van der Waals forces than in chlorine.
4. Phosphorus: Phosphorus can take several forms (white, red, black), but white phosphorus has a relatively low melting point; however, when considering the solid form, phosphorus has stronger covalent bonds than sulfur, leading to a higher melting point.

Based on this information, we conclude that:

$\text{Melting Point Order: } \text{Sulfur} > \text{Phosphorus} > \text{Chlorine} > \text{Argon}$

Thus, the correct order of melting points is represented by option C: sulfur > phosphorus > chlorine > argon.