Amongst the elements with following electronic configurations, which one of them may have the highest ionisation energy ? (AIPMT 2009) (a) Ne 3s 3p (b) Ar 3d 4s 4p (c) Ne 3s 3p (d) Ne 3s 3p

Among options (a), (c) and (d), option (d) has the highest ionisation energy because of extra stability associated with half-filled 3p-orbital. In option (b), the presence of 10 3d electrons offers shielding effect, as a result the 3 4p electrons do not experience much nuclear charge and hence, the electrons can be removed easily.