Molecular size affects van der Waals forces and melting points Steps: - Observe melting point trend: rises from P (44°C) to S (119°C), then falls sharply for Cl (-101°C) and Ar (-189°C). - Identify molecular formulas: P exists as P₄ (tetrahedral), S as S₈ (crown-shaped), Cl as Cl₂ (diatomic), Ar as monatomic Ar. - Recognize that larger molecules (P₄ and S₈) have greater electron clouds, leading to stronger intermolecular van der Waals forces and higher melting points. - Conclude that the variation in atoms per molecule directly correlates with the observed melting point changes. Why D is correct: - Larger molecules increase surface area and polarizability, strengthening van der Waals forces (temporary dipoles) that require more energy to overcome for melting. Why the others are wrong: - A: Electronegativity changes influence bond polarity but do not account for the non-linear melting point trend across these nonpolar or weakly polar molecules. - B: First ionisation energy trends relate to atomic stability, not intermolecular forces determining melting points in these molecular solids/gases. - C: Electron number increases steadily across the period but …