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AB forms a body-centred cubic crystal lattice with unit-cell edge length a = 387 pm. What is the separation between two ions of opposite charge in the lattice?

1. 335 pm
2. 250 pm
3. 200 pm
4. 300 pm

Correct answer: 335 pm

Solution

For a body-centred (CsCl-type) AB lattice the nearest opposite ions are half the body diagonal: sqrt3 * 387 / 2 = 335 pm, not 250 pm.

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