The atomic radius of Halogens is smaller than the ionic radius.

This is because they react by gaining/acquiring extra one electron in the outer energy level.

The effective nuclear attraction on the more/extra electrons decreases.

The incoming extra electron is also repelled causing the outer energy level to expand to reduce the repulsion and accommodate more electrons.

The ease of gaining/acquiring extra electrons is called electronegativity.

 All halogens are electronegative. Electronegativity decreases as atomic radius increase.

This is because the effective nuclear attraction on outer     electrons decreases with increase in atomic radius