Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : The first ionization enthalpy for oxygen is lower than that of nitrogen.
Reason R : The four electrons in 2p orbitals of oxygen experience more electron-electron repulsion.
In the light of the above statements, choose the correct answer from the options given below.

Solution

Chemistry ma’am · 4 steps

Compare the 2p fillings: nitrogen is a calm half-filled set, oxygen has one crowded orbital.

1. Check the assertion. The first ionisation enthalpy of N is about 1402 kJ mol⁻¹ and that of O is about 1314 kJ mol⁻¹. So oxygen is lower, even though it comes later in period 2. A is correct.
2. Nitrogen, 2p³. In nitrogen, the three 2p electrons sit one in each orbital with parallel spins. No orbital holds a pair, so repulsion is low and this half-filled set is extra stable.
3. Oxygen, 2p⁴. In oxygen, the fourth 2p electron must pair up in an orbital that already has one electron. Two electrons in the same region of space repel strongly, which raises the energy of that paired electron and makes it easier to remove.
4. So. R is correct, and the extra electron-electron repulsion in oxygen's 2p subshell is exactly why A happens. The answer is (a).

The idea to keep. When ionisation enthalpy dips across a period, look at the electron being removed: a paired electron (as in O) or a new subshell (as in B) comes off more easily.

Answer from the NTA final answer key. Solution by PrepWiser.