NiO adopts a rock-salt structure. The coordination number of the Ni+2 ion is

NiO has cubic structure, so Ni+2 has 6 coordination number. Rock salt follows 6 : 6 type arrangement.C.NO. of Na+ ion =6  and octahedral voids are occupied by Na+ ion.C.NO of Cl- ion = 6 and Cl-ions follows fcc arrangement.