Periodic trends and compound properties

Steps:

• Locate Na (period 3, group 1) and Si (period 3, group 14) in periodic table.
• Evaluate A: Na₂O reacts with water to form NaOH (basic, pH >7); SiO₂ is insoluble and neutral (pH ≈7).
• Evaluate B: In NaCl, Na oxidation state is +1; in SiCl₄, Si is +4.
• Evaluate C: Atomic radius decreases across period 3 due to increasing nuclear charge.
• Evaluate D: NaCl (ionic lattice) has high melting point (~801°C); SiCl₄ (covalent molecular) has low melting point (-70°C).

Why C is correct:

• Atomic radius decreases left to right in a period as effective nuclear charge increases, pulling electrons closer (Na radius 186 pm > Si 111 pm).

Why the others are wrong:

• A: Na₂O gives higher pH (basic) than SiO₂ (neutral).
• B: Na oxidation state (+1) is lower than Si (+4).
• D: NaCl melting point is higher than SiCl₄.

Final answer: C