Halogens are elements found in the 7-th group on the periodic table. They are predominantly know for their electronegativity values and they have a high ionization energy.
Ionization energy is the energy required to remove the most loosely held electrons in an atom.
Across a period, ionization energy is known to increase progressively from left to right.
This due to the decreasing atomic radii caused by the increasing nuclear charge which is not compensated for by successive electronic shells.