Group 17

Halogens and halides

Down group 17, halogens become darker and less volatile (stronger London forces), electronegativity and oxidising power decrease, while halide ions become stronger reducing agents (larger ions lose electrons more easily). A more reactive halogen displaces a less reactive halide. Silver nitrate test (acidified with HNO₃): Cl⁻ white (dissolves in dilute NH₃), Br⁻ cream (concentrated NH₃), I⁻ yellow (insoluble). With concentrated H₂SO₄, NaCl gives only HCl; NaBr gives Br₂ and SO₂; NaI gives I₂, S and H₂S. Chlorine disproportionates in water and in cold or hot NaOH.