Q.The values of first ionisation enthalpy of S and Cl are 1000 and 1256 kJ mol$^{-1}$, respectively. Explain the observed trend.

Group 17, the halogens (F, Cl, Br, I, At), has the general valence-shell configuration ns2np5 and is named from the Greek halo (salt) + gene (born), since halogens are extremely reactive and always found combined, never free, because of their very high electronegativities. Fluorine occurs as fluorspar, cryolite and fluorapatite; chlorine, bromine and iodine occur mainly as seawater/evaporite chlorides, bromides and iodides (notably NaCl and carnallite); astatine is intensely radioactive with a short half-life. Down the group, atomic and ionic radii increase, ionisation enthalpy decreases, and electronegativity decreases -- halogens have the smallest atomic radii of any group in their own period, and fluorine is the single most electronegative element in the periodic table. Electron gain enthalpy becomes less negative down the group overall, but fluorine is an exception (like oxygen in group 16), being LESS negative than chl …