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Select the CORRECT ionisation energy order from the following options.

1. Ag < Cu < Au
2. O < N < F
3. La < Y < Sc
4. All of these

Correct answer: La < Y < Sc

Solution

Only the order La < Y < Sc is correct because ionization energies in Group 3 increase going up the group (La has the lowest IE, Sc the highest). The order Ag < Cu < Au is incorrect because Cu actually has lower IE than Ag, and Au (due to relativistic effects) has very high IE — but the specific ranking Ag < Cu < Au is wrong (correct order is Ag < Au < Cu or similar). The order O < N < F is wrong because N has higher IE than O (half-filled 2p stability).

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