Question

Assertion: The unit cell of a face-centred cubic system contains 4 atoms.

Reason: The unit cell of a face-centred cubic system consists of atoms at face centres in addition to atoms at the corners.

Options

• If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion.

• If both Assertion and Reason are CORRECT but Reason is not the CORRECT explanation of the Assertion.

• If Assertion is CORRECT but Reason is INCORRECT.

• If Assertion is INCORRECT but Reason is CORRECT.

Answer 1

If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion.

Explanation:

In a face-centred cubic (FCC) unit cell, atoms are present at 8 corners and 6 face centres.
Each corner atom contributes `1/8` and each face-centred atom contributes `1/2` to a unit cell.
So, total atoms = `8 xx 1/8 + 6 xx 1/2`

= 1 + 3

= 4
Thus, the Reason correctly explains the Assertion.