Halogens exhibit low melting points due to weak intermolecular forces

Steps:

• Identify four consecutive elements with atomic numbers <20 including Group 17: phosphorus (15), sulfur (16), chlorine (17), argon (18).
• Recall melting point trend: phosphorus (44°C) and sulfur (115°C) higher due to network/covalent structures; chlorine (-101°C) low as diatomic gas; argon (-189°C) lowest as noble gas.
• Analyze graph: shows rise to sulfur, sharp drop at chlorine, further drop at argon.
• Match options to positions: A (P), B (S), C (Cl), D (Ar); C shows the halogen's distinct low point.

Why C is correct:

• Chlorine (Group 17) has low melting point from weak van der Waals forces in Cl₂ molecules, per periodic trends.

Why the others are wrong:

• A: Phosphorus in Group 15, higher melting point from polymeric structure.
• B: Sulfur in Group 16, highest melting point from S₈ rings.
• D: Argon in Group 18, low but not Group 17; monatomic noble gas.

Final answer: C