The electronic configuration of elements are as follows Li=1s22s1;‌‌Bc=1s22s2 B=1s22s22p1;‌‌N=1s22s22p3 All these element belong to second period. The ionisation energy generally increases as you move from left to right across a period. The first ionisation energy of beryllium is greater than that of boron since the order of attraction of electrons towards nucleus is 2s>2p. The beryllium has a stable complete electronic configuration 1s22s2 whereas boron has the electronic configuration 1s22s22p1 which needs lesser energy than that of beryllium to remove the valence electron. Therefore, the order of first ionisation energy is Li<B<Be<N.