Question

Which one of the following elements has the least ionisation energy?

• A. Caesium
• B. Lithium
• C. Sodium
• D. Potassium

Answer 1

The correct option is A

Caesium

Ionisation energy is the minimum amount of energy required to remove the most loosely bound electron i.e the valence electron, of an isolated neutral, gaseous, atom.

Among the given choices, caesium has the largest atomic radius and maximum metallic character. Hence, it is easiest to extract an electron from it, i.e., it has the least ionisation energy.