Question

The coordination number of FCC structure for metals is $12$ , since:
(A) Each atom touches $4$ other in the same layer, $3$ in layer above and $3$ in layer below.
(B) Each atom touches $4$ other in the same layer, $4$ in layer above and $4$ in layer below.
(C) Each atom touches $6$ other in the same layer, $3$ in layer above and $3$ in layer below.
(D) Each atom touches $3$ other in the same layer, $6$ in layer above and $6$ in layer below.

Answer 1

In FCC structure, one constituent atom is present at each corner of the cube and one atom at each center of its face. Coordination number of an atom is defined as the number of atoms, molecules or ions attached to it.

Complete answer:
In FCC structure, the atoms are present at each corner and each face center as shown in the following diagram.

If we consider the face centred atom of the cube in FCC structure then we see that each face centred atom is surrounded by $4$ corner atoms of its own layer and $4$ face centred atoms of its adjacent upper layer as well as $4$ adjacent lower layer. Therefore, the total number of atoms it is surrounded by is $4 + 4 + 4 = 12$ . Hence, the coordination number of FCC structure for metals is $12$ .
This proves that option B is correct.

Note:
Before attempting the question, try to imagine the structure of an FCC unit cell with precise location of the atoms. Also remember that there are $8$ corner atoms and $6$ central atoms in one FCC unit cell.