2022 O Level Chem P1 Q6 — Ionic Bonding Solution

Answer: D. Sodium chloride's giant ionic lattice is built on a 6:6 coordination scheme: every \(\text{Na}^+\) is directly surrounded by 6 \(\text{Cl}^-\) ions (one above, one below, and four around in the same plane), and every \(\text{Cl}^-\) is in turn surrounded by 6 \(\text{Na}^+\) ions in the same way.

The diagram shows only a small unit of the lattice; the structure repeats in all three dimensions. Even though only a few \(\text{Cl}^-\) ions appear near a given \(\text{Na}^+\) in the snippet drawn, the full lattice puts six chloride ions in direct contact with each sodium ion.

3, 4 and 5 are all too low; 6 is the correct coordination number for both ions in the rock-salt structure that NaCl adopts. Answer D.