Question

Arrange the following elements in the increasing order of first ionisation enthalpies.

• A. Sodium (Na)
• B. Caesium (Cs)
• C. Lithium (Li)
• D. Potassium (K)

Answer 1

All these are alkali metals and belong to first group of the periodic table. Li at the top and Cs at the bottom.
The atomic size increases from Li to Cs. As a result, the effect of nuclear charge on the outermost electron of Cs is least and hence, it can be easily removed.
The correct order of first ionisation enthalpy would be:
Cs < K < Na < Li