Question

Cobalt metal crystallizes in a hexagonal closest packed structure. What is the coordination number of a cobalt atom?

a) 4
b) 6
c) 8
d) 12

Answer 1

The coordination number of a cobalt atom in a hexagonal closest packed structure is d. 12, with each atom contacting six atoms in its own layer, and three atoms each in the layers above and below.

Step-by-step explanation:

The coordination number of a cobalt atom in a hexagonal closest packed (hcp) structure is d. 12. In an hcp structure, each atom contacts six atoms in its own layer, three in the layer above, and three in the layer below, making the total number of nearest-neighbor atoms 12. This pattern results in an ABABAB stacking sequence and is common to many metals including Cd, Co, Li, Mg, Na, and Zn.

The hexagonal closest-packed structure is known for maximizing attractions between atoms and minimizing intermolecular energy, a characteristic shared with the cubic closest-packed structure (CCP), which also has a coordination number of 12 but consists of an ABCABC stacking sequence.