Question

The correct order of second ionisation potential of carbon, nitrogen, oxygen and fluorine is (J1981,1M)

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

Answer 1

The correct option is C O > F > N > C

For second ionisation potential, electron will have to be removed from valence shell of the following ions:

In general, ionsiation energy increases from left to right in a period. However, exception occur between adjacent atoms in a period, greater amount energy is required for removal of electron from completely half -filled or completerly filled orbital than the same for adjacent atom with either less than completely half-filled or less than completely filled orbital. Therefore, ionisation potential of $O^{+}$ is greater than that of $F^{+}$. Also ionisation potential of $N^{+}$ is greater than $C^{+}$ but less than both $O^{+}$ and $F^{+}$ (periodic trend). Hence overall order is 2nd IP : O > F> N > C.