Question

Assertion (A): The first ionization enthalpy of nitrogen is higher than that of oxygen.
Reason (R): Nitrogen has a half-filled \(2p\) orbital, which is extra stable.

• A. A and R are both true, and R is the correct explanation of A.
• B. A and R are both true, but R is not the correct explanation of A.
• C. A is true, but R is false.
• D. A is false, but R is true.

Hint:

Remember the important exception in ionization enthalpy: \[ \boxed{IE_1(N)\gt IE_1(O)} \] because \[ \boxed{N:2p^3} \] is a stable half-filled configuration, whereas oxygen has one paired electron in the \(2p\) subshell, making electron removal easier.

Answer 1

The Correct Option is A

Concept: Ionization enthalpy is the minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom. Generally, ionization enthalpy increases across a period due to increasing effective nuclear charge. However, nitrogen and oxygen show an exception because of the stability associated with half-filled orbitals. Nitrogen has the electronic configuration \[ 1s^22s^22p^3, \] which is a perfectly half-filled \(2p\) subshell. Half-filled and completely filled subshells possess extra stability due to symmetrical distribution of electrons and maximum exchange energy. Oxygen has the electronic configuration \[ 1s^22s^22p^4, \] where one of the \(2p\) orbitals contains a pair of electrons. The repulsion between paired electrons makes it comparatively easier to remove one electron.

Step 1: Examine the assertion.
The first ionization enthalpy of nitrogen is greater than that of oxygen. \[ IE_1(N)\gt IE_1(O) \] This statement is true because removing an electron from the stable half-filled configuration of nitrogen requires more energy.

Step 2: Examine the reason.
Nitrogen possesses the electronic configuration \[ 2p^3, \] which is a half-filled subshell. Half-filled subshells are extra stable because of maximum exchange energy and symmetrical electron distribution. Hence, the reason is also true.

Step 3: Determine whether the reason explains the assertion.
The higher stability of nitrogen due to its half-filled \(2p\) subshell directly explains why its first ionization enthalpy is higher than that of oxygen. Therefore, \[ \boxed{\text{Assertion is true, Reason is true, and Reason correctly explains the Assertion.}} \] Hence, the correct answer is \[ \boxed{\textbf{Option (A)}}. \]