The general electronic configuration of halogens is np⁵, where n = 2-6. Thus, halogens need only one more electron to complete their octet and to attain the stable noble gas configuration. Also, halogens are highly electronegative with low dissociation energies and high negative electron gain enthalpies. Therefore, they have a high tendency to gain an electron.

Whenever an atom accepts an electron from another atom, the atom accepting the electron is getting reduced and the atom donating the electron is oxidized. But the electron accepting atom acts as an oxidizing agent while the electron donating atom acts as reducing agent.

Hence, they act as strong oxidizing agents.