Question

Why the first ionization energy of N is lower than the second ionization energy of O?

Answer 1

The second ionization energy of oxygen is higher than for nitrogen. ... It is because oxygen atom is smaller due to a higher effective nuclear charge Zeff=Z−S , where S is approximated to be the number of core electrons and Z is the atomic number.

Answer 2

After first ionization OF attains

stable configuration with half

filled 2p orbital whereas after 1st

ionization N loses its stability

due to half filled 2p orbital.

Hence, now to remove another

electron from A will require more

energy as its stable now, where

the task will be easy for N as it

has lost it stability.