In a NaCl crystal, each sodium ion is surrounded by 6 Cl^-(Chloride) ions nearest neighbours and ________ Na⁺ (Sodium) ions next nearest neighbours.

12

The answer is 2. 12.

In a NaCl crystal, each sodium ion is surrounded by 6 chloride ions as nearest neighbors. The next nearest neighbors are 12 sodium ions.

This is because the NaCl crystal has a face-centered cubic (fcc) structure. In an fcc structure, each atom is surrounded by 12 other atoms, 6 of which are nearest neighbors and 6 of which are next nearest neighbors.

The nearest neighbors are the atoms that are located directly next to the central atom. The next nearest neighbors are the atoms that are located one layer away from the central atom.