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What is the correct sequence of coordination numbers for simple cubic (SC), face-centred cubic (FCC), and body-centred cubic (BCC) crystal structures?

1. 6, 8, 12
2. 6, 12, 8
3. 8, 12, 6
4. 8, 6, 12

Correct answer: 6, 12, 8

Solution

SC: Each atom at a corner is touched by 6 atoms (one along each of +x, -x, +y, -y, +z, -z). Coordination number = 6. FCC: Each atom is surrounded by 12 nearest neighbours (4 in its own layer, 4 above, 4 below). Coordination number = 12. BCC: The atom at body centre has 8 nearest neighbours at the 8 corners, and vice versa. Coordination number = 8. Sequence SC, FCC, BCC = 6, 12, 8.

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