Explanation (Detailed Concept)

Statement I:

Across a period, atomic radius decreases from left to right due to increase in effective nuclear charge.

Therefore, in Period 3:

Mg > Al

Hence the correct atomic order is:

Mg > Al

For cations:

More positive charge → smaller radius.

Thus:

Mg²⁺ > Al³⁺

Combining all correctly:

Mg > Al > Mg²⁺ > Al³⁺

But the given order is:

Al > Mg > Mg²⁺ > Al³⁺

This is incorrect.

Statement I is false.

Statement II:

Electron gain enthalpy generally becomes more negative across a period and less negative down a group.

Among halogens:

Cl has more negative electron gain enthalpy than Br.

Between O and S:

S has more negative electron gain enthalpy than O due to smaller inter-electronic repulsion in larger 3p orbital.

Thus correct order (magnitude):

Cl > Br > S > O

This matches the given statement.

Statement II is true.

Final Answer: Statement I is false but Statement II is true.

Correct Option: C

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