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

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

IIT JEEIIT JEE 1981Classification of Elements and Periodicity in Properties

Solution:

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