Half filled orbitals and fully filled orbitals are more stable.
(i) Atomic no. of Be=4→1s22s2
Atomic no. of B=5→1s22s22p1
In case of Be it has fully filled s−orbital thus it is more stable and more ionization enthalpy is needed to remove an electron from it.
In case of B, the outermost electron is in p−orbital , after removal of which it become stable with stable configuration 1s22s2
Thus B need less energy to remove the outermost electron
Hence I.E of B< I.E of Be
Atomic no. of O=8→1s22s22p4
Atomic no. of N=7→1s22s22p3
In case of N its p−orbital is half filled which is more stable than other configuration, thus it required more energy to remove the outermost electron it become more stab;e with configuration
Thus it required less energy to remove the outermost electron and hence
I.E of O< I.E of M
and because of high electronegativity of F, it is also difficult to remove the outermost electron of F.
Therefore , I.E of F> I.E of O