Question

The coordination number of hexagonal close packing (hcp) structure is:
A.12
B.10
C.8
D.6

Answer 1

The concept of coordination number can be understood as the number of atoms or ions that are attached to any given particular atom. This property is also known as ligancy. It can be determined by observing the structure under consideration.
Complete Step-by-Step answer:

Complete step by step answer:
Hexagonal close packing is a packing structure in which the lattice layers are arranged in the form of ABAB, where each letter signifies an individual lattice configuration and a collection of these lattices forms a closed packing structure.
In hexagonal close packing, the top and the bottom lattice are similar with the geometry of a hexagon. Atoms are placed on the vertices and in the centre of the hexagon. And the middle layer has a triangular geometry, where the atoms are placed on the vertices of the triangle.
Because of such a geometry, any atom in the given hexagonal close packing structure would be surrounded by 4 atoms in the same lattice as that of the concerned atom, by 4 atoms in the lattice above the atom and by 4 atoms in the lattice below the atom.
That can be simplified by saying that any given atom in hexagonal close packing is attached to 4 + 4 + 4 = 12 atoms at any point in time. hence by definition, the coordination number of hcp is 12.

Hence, Option A is the correct option.

Note:
The coordination number of any type of unit cell is a characteristic property of that structure and never changes in any condition. Although, we may experience certain anomalies while explaining the coordination number when explaining crystal defects.