Question

The correct order of second ionisation potential of carbon, nitrogen, oxygen and fluorine is

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

Answer 1

Answer: (C)

O > F > N > C

Answer 2

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