OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 5

✅ Correct Answer

D (Ar)

Argon exists as single, isolated atoms (monatomic). It has the smallest electron cloud of the choices given, resulting in the weakest London forces (induced dipole-dipole interactions), which require the least thermal energy to overcome.

💡 Key Knowledge

• Period 3 structures: Na, Mg, Al are metallic; Si is giant covalent; P₄, S₈, Cl₂ are simple molecular; Ar is monatomic.
• Intermolecular forces: Simple molecular and monatomic substances are held together by weak London forces.
• Trend: Melting point depends on the size of the molecule/atom and the number of electrons, which determines the strength of the London forces. Ar has only 18 electrons and exists as single atoms.

🧠 Exam Technique

Quickly categorise the elements listed in the options. Realise that sulfur (S₈), phosphorus (P₄), and chlorine (Cl₂) form larger molecular lattices with more electrons than a single argon atom (Ar), meaning argon has the smallest relative molecular/atomic mass and weakest instantaneous dipole-induced dipole forces.

❌ Common Errors

• Confusing melting point trends with atomic radius or electronegativity.
• Assuming that because chlorine is a gas at room temperature, it automatically has a lower melting point than solid sulfur, without considering that argon is a noble gas existing as individual atoms with even fewer electrons than Cl₂.