Question

A solid is made up of two elements P and Q. Atoms Q are in hcp arrangement while atoms P occupy all the tetrahedral sites. What is the formula of the compound?

Answer 1

Q atoms are in a hexagonal close-packed (hcp) arrangement.

P atoms occupy all the tetrahedral voids.

In the hcp arrangement, the number of atoms (Q) per unit cell = 6

The number of tetrahedral voids in any close-packed structure, whether it is hcp or ccp, is calculated as follows:

Number of tetrahedral voids = 2 × number of atoms in the close-packed structure

Number of P atoms (in tetrahedral sites) = 2 × 6 = 12

∴ Empirical formula = P : Q = 12 : 6

= 2 : 1

∴ The formula of the compound = P₂Q