Explain the trends in the following properties of group 17 elements : (i) Atomic radii (ii) Electronegativity

Answer: Down group 17, atomic radii increase (F < Cl < Br < I) and electronegativity decreases (F > Cl > Br > I).

(i) Atomic radii: Halogens have the smallest atomic radii in their respective periods because of high effective nuclear charge. Down the group, a new shell is added at each step, so the atomic radius increases from fluorine to iodine.

(ii) Electronegativity: Halogens are highly electronegative because of their small size and the need for only one electron to complete the octet. Down the group, atomic size increases and the attraction of the nucleus for the shared pair decreases, so electronegativity decreases from F to I. Fluorine (4.0 on Pauling scale) is the most electronegative element.