Question

How do you tell how many ions are in the unit cell of BaO?

Answer 1

Barium oxide has rock salt type of structure , that is like NaCl. Thus four ion pairs of Ba+2 and O-2 would be present in unit cell and smaller oxide ions occupying all octahedral voids and larger Barium ions forming the lattice . Thus it would have 13 oxide ions and 14 barium ions in one unit cell, however as per the contribution to each unit cell

the number of Ba⁺² ions = 8x1/8 + 6x1/2 = 4 and the number of O^-2 ions = 1 + 6x1/2 = 4

Since the ratio of Ba+2 to O-2 is 1:1 the empirical formula is BaO.