Question

The order of decreasing ionisation enthalpy in alkali metals is

• A. Rb < Na < K < Li
• B. K < Li < Na < Rb
• C. Na > Li > K > Rb
• D. Li > Na > K > Rb

Answer 1

The correct option is D Li > Na > K > Rb

Solution:

Definition of ionisation enthalpy

The minimum amount of energy required to remove the outermost shell electron from an isolated gaseous atom is called ionisation enthalpy.

Relation between atomic size and ionisation enthalpy-

The smaller the size of the atomic radii, the more energy is required to remove the valence electron. Down the group of alkali metals from Li to Rb, size increases. As size increases, effective nuclear charge faced by the outermost shell electrons decrease, thus ionization energy decreases.

Thus, the decreasing order of ionization enthalpy in alkali metals is Li > Na > K > Rb.

​​​​​​​The correct option is (C).