CCl₄ exists but PbCl₄ is unstable; NCl₃ hydrolyses but PCl₃ also does — yet CCl₄ doesn't hydrolyse while SiCl₄ does violently. Explain. Step 1: C (2nd period) has max covalence 4, no d orbitals — water can't attack the crowded carbon. Step 2: Si (3rd period) expands octet using d orbitals; water attacks → violent hydrolysis. Step 3: NCl₃ hydrolyses (small N, polar N–Cl), PCl₃ too (vacant d orbitals accept water). Step 4: Second-period rule: no d orbitals = no expansion, different chemistry!