Nonmetals. The halogens

The halogens are fluorine F, chlorine Cl, bromine Br, iodine I and astatine At; they stand in the main subgroup of group VII and have 7 electrons in the outer shell. So each accepts one electron and shows the oxidation state –1; fluorine is –1 in all compounds, while chlorine, bromine and iodine range from +1 to +7 in oxygen compounds. As free elements they are very active, so in nature they occur only in compounds: CaF₂ (fluorspar), NaCl and KCl, NaBr, KI and others. From fluorine to iodine the atomic radius grows, the colour deepens, the state changes from gas to liquid to solid, and melting and boiling points rise: fluorine is a pale yellow gas, chlorine a yellowish-green gas, bromine a reddish-brown liquid, iodine a dark grey crystal. The oxidizing power falls from F₂ to I₂, so a more active halogen displaces a less active one from its salt: Cl₂ + 2KBr → 2KCl + Br₂. Iodine on heating turns into violet vapour without melting — this is sublimation. Halogen vapours are poisonous and must never be smelled.