Question

which one has highest first ionisation energy N or O​

Answer 1

Answer:

Nitrogen has highest first ionisation energy

Answer 2

Answer:

O has a lower 1st IE than N because O is 1s2 2s2 2p4 and N is 1s2 2s2 2p3. The 4th electron in the O atom's outer level has to pair up with one of the others, and this introduces repulsions, which makes it easier to remove. The relevant factors are nuclear charge, atomic radius and screening by inner electrons.