Question

Correct order of 2$^{nd}$ ionisation energy is:

• A. O<C<N<F
• B. C<N<O<F
• C. O<C<N<F
• D. C<O<N<F

Hint:

Half-filled and fully-filled subshells show extra stability, affecting ionisation energy trends.

Answer 1

The Correct Option is C

Step 1: Write electronic configurations after first ionisation.
\[ O \rightarrow O^+ (2p^3) \] \[ N \rightarrow N^+ (2p^2) \] \[ C \rightarrow C^+ (2p^1) \] \[ F \rightarrow F^+ (2p^4) \]
Step 2: Analyze stability of configurations.
Half-filled $2p^3$ configuration is most stable.
Completely filled and half-filled orbitals show extra stability.
Step 3: Compare second ionisation energies.
Removing an electron from stable $2p^3$ requires more energy than from $2p^4$.
Thus, IE increases from left to right but irregular due to electronic stability.
Step 4: Final order.
\[ O<C<N<F \]