KEAM 2011 Physics and Chemistry Paper

In general, as we move from left to right in a period, the ionisation enthalpy increases with increasing atomic numbers. But the ionisation enthalpy of B is lower than that of Be. This is because the electronic configuration of B(1s22s22p1) is less stable than that of Be (1s22s2) which has completely filled orbitals. As a result, the 2p -electron of B is not strongly attracted by the nucleus as the 2 -electron of Be. Thus, the increasing order of first ionisation enthalpies of elements is F>N>C>Be>B