The correct increasing order of the second ionisation enthalpies for the elements C , N , O and F is:

Step-by-step solution

The second ionisation enthalpy is the energy required to remove an electron from the unipositive ion ( M ^+ ). The electronic configurations of the unipositive ions are: C ^+: 1s^2 2s^2 2p^1 N ^+: 1s^2 2s^2 2p^2 O ^+: 1s^2 2s^2 2p^3 F ^+: 1s^2 2s^2 2p^4 Generally, ionisation enthalpy increases across a period from left to right. However, O ^+ has a highly stable half-filled 2p subshell ( 2p^3 ), which requires exceptionally high energy to remove an electron. As a result, the second ionisation enthalpy of oxygen is