Halogens belong to group 17 of the periodic table. The justification in placing them in 17 group is their similar properties:
(i) All of them are non-metals and non-metallic nature decreases with the increase in atomic number.
(ii) They are strongly electronegative elements and electronegativity decreases with the increase in atomic number.
(iii) Melting and boiling points increase with the increase in atomic number.
(iv) They have outer electronic configuration of ns2 np5.
They, therefore, form a group and must be placed before noble gas elements in the p-black. Their placement in 17th group is thus justified.