Be – 1s²2s²;

B – 1s²2s²2p¹;

C – 1s²2s²2p²;

N – 1s²2s²2p³;

O – 1s²2s²2p⁴.

IP increases along the period. But IP of Be > B. Further IP of O < N because atoms with fully or partly filled orbitals are most stable and hence have high ionisation energy.

As in case of boron, 2p¹ electron have to be removed to get B⁺(1s² 2s²) from B(1s² 2s² 2p¹), while in case of Be (1s²2s²) electron have to be removed to get Be⁺ (1s²2s¹). p electron can be removed more easily than s electron so the energy required to remove electron will be less in case of boron.