Which of the following has highest ionisation energy ?

Options

A. Oxygen
B. Nitrogen
C. Fluorine
D. Sulphur

Answer

D

Explanation

Among oxygen, nitrogen, fluorine and sulphur, the lowest ionisation energy is of sulphur.
As their first ionisation values indicated below.
N=1402.1 kJ/mol, O=1313.7 kJ/mol,
F=1680.8 kJ/mol, S=999.4 kJ/mol.
As down the group, the number of inner shells increases. Therefore ionisation energy decreases due to screening effect.