Q.Which one of the following orders is correct for the bond dissociation enthalpy of halogen molecules? (NEET)

Group 17, the halogens (fluorine, chlorine, bromine, iodine, astatine), share the outer configuration ns2 np5, one electron short of a filled octet, which makes them the most reactive non-metals and ensures they are never found free in nature. Fluorine's main source is fluorspar (CaF2, plus cryolite and fluorapatite); chlorine's is sodium chloride from sea water/salt deposits; bromides and iodides also occur dissolved in sea water.

Down the group, physical state changes from gas (F, Cl) to liquid (Br) to solid (I, At), while atomic radius, density, melting point and boiling point all increase steadily as heavier, more polarisable atoms are reached. Bond dissociation enthalpy and oxidising power, however, do NOT fall monotonically from fluorine: chlorine's Cl-Cl bond is in fact stronger than fluorine's unusually weak F-F bond (small fluorine atoms suffer significant lone-pair-lone-pair repulsion across the short F-F bond), so the order is Cl2 > F2 > Br2 > I2 for bond dissociation enthalpy, even though fluorine remains the single strongest oxidising agent and the most reactive element of the group overall …