Question

Assertion (A): The first ionisation energy of aluminium is lower than that of magnesium.

Reason (R): lonic radius of aluminium is smaller than that of magnesium.

Options

• If both A and R are true, and R is the correct explanation of A.

• If both A and R are true, but R is not the correct explanation of A.

• If A is true, but R is false.

• If both A and R are false.

Answer 1

If both A and R are true, but R is not the correct explanation of A.

Explanation:

The first ionisation energy of aluminium is lower than that of magnesium because aluminium loses its outermost electron from a higher-energy, less stable 3p orbital, whereas magnesium requires more energy to remove an electron from a fully filled, highly stable 3s orbital. Separately, the ionic radius of aluminium (Al³⁺) is smaller than that of magnesium (Mg²⁺) due to a higher effective nuclear charge pulling its remaining electrons closer to the nucleus. While both statements are individually true, the difference in ionic size does not explain the initial electronic configuration anomaly responsible for their respective ionisation energies.