Question

The correct order of the 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)

$O>F>N>C$

The electronic configurations of ions after first ionization are:
$O^{+}:1s^{2}2s^{2}2p^3$
$F^{+}:1s^{2}2s^{2}2p^4$
$N^{+}:1s^{2}2s^{2}2p^2$
$C^{+}:1s^{2}2s^{2}2p^1$
As we go across a period, Zeff increases because valence electrons do not screen the nuclear charge effectively. Hence, the IP's increase across a period. But, oxygen has more second ionisation potential than fluorine. This is because after the first ionisation, oxygen acquires stable half filled configuration. Thus, for the second ionisation, electron has to be removed from $2p^3$ sub-shell. While in the case of fluorine, it is getting removed from $2p^4$ sub-shell.