A solid is formed of two different elements, A and B. In this atoms...

In a CCP lattice, the number of atoms per unit cell is $4$.

Number of atoms of A $= 4$

The number of octahedral voids is equal to the number of atoms in the lattice, which is $4$.

The number of tetrahedral voids is twice the number of atoms in the lattice, which is $8$.

Atoms of B occupy all octahedral voids and half of the tetrahedral voids.

Number of atoms of B $= 4 + \dfrac{1}{2} \times 8 = 4 + 4 = 8$

The ratio of atoms of A to atoms of B is $4 : 8 = 1 : 2$.

The formula of the solid is $\text{AB}_2$.

Answer: $\text{AB}_2$