The order of decreasing ionisation enthalpy in alkali metals is
(a) Na> Li > K> Rb (b) Rb< Na< K< Li
(c) Li > Na> K> Rb (d) K< Li < Na < Rb
Answer: (c) Li > Na> K> Rb
On moving down in the group (from Li to Cs) the ionization energy value decreases from Li to Cs, size of the atom increases and so valence electron is less tightly held increased screening effect from Li to Cs also makes the removal of electron easier. Li has more ionization energy than Na than K than Rb.