lionization energy generally increase from left to right in a period. Hence expected increasing order is Be, B, N, O but the actual order, found is: B < Be < O < N The reason is that Be and N, have comparatively stabler electronic configurations ( $ n{{s}^{2}} $ and $ n{{p}^{3}} $ ) than those of B and O ( $ n{{p}^{1}} $ and $ n{{p}^{4}} $ ). Hence, their ionisation energy becomes higher.