Question

The co-ordination number of fcc structure for metals is 12, since

• A. Each atom touches 4 others in same layer, 3 in layer above and 3 in layer below.
• B. Each atom touches 4 others in same layer, 4 in layer above and 4 in layer below.
• C. Each atom touches 6 others in same layer, 3 in layer above and 3 in layer below.
• D. Each atom touches 3 others in same layer, 6 in layer above and 6 in layer below.

Answer 1

Answer: (B)

Each atom touches 4 others in same layer, 4 in layer above and 4 in layer below.

Answer 2

Fcc can be viewed in two following ways –

(i) Planes along the faces (and parallel to it) of the unit cell.

$\Rightarrow$Each atom touches 4 in same layer, 4 in layer above and 4 in layer bellow it.

(ii) Planes along closest packed spheres $\rightarrow$ each atom touches 6 atom in same layer, 3 in layer above and 3 in layer below it.