Group 17: the halogen column, top to bottom

Seven outer electrons: ns² np⁵

Each configuration ends in ns² np⁵, from fluorine ([He] 2s² 2p⁵) to astatine ([Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵). One more electron completes the octet, so the usual charge is −1, as in F⁻, Cl⁻ and I⁻, and in molecules each atom typically forms one covalent bond.

Fluorine, the most electronegative element, is always −1. Bonded to oxygen, the heavier members take positive oxidation states: chlorine is +1 in the hypochlorite of bleach (ClO⁻) and +7 in perchlorate (ClO₄⁻), and iodine also reaches +7.

Reading the trends in numbers

Electronegativity falls steadily down the group: 3.98 for fluorine, 3.16 for chlorine, 2.96 for bromine, 2.66 for iodine and 2.2 for astatine. First ionization energy drops from 17.423 eV for fluorine to 10.451 eV for iodine.

Melting and boiling points rise as the molecules get bigger, and they predict each element’s state at room temperature, about 298 K. Chlorine boils at 239.11 K, so it is a gas. Bromine melts at 265.8 K and boils at 332 K, so it is a liquid. Iodine does not melt until 386.85 K, so it is a solid.

Where you meet group 17

Chloride ions help balance your body’s fluids and form the hydrochloric acid in your stomach, and your thyroid needs iodine to make its hormones. About a fifth of modern medicines contain fluorine. CFCs, chlorine and fluorine compounds once used in spray cans and refrigerators, damaged the ozone layer and were phased out under the 1987 Montreal Protocol.

Group 17 versus the halogens

The halogens family on this site lists five elements, because tennessine (117) is too little known to classify with confidence. First made in 2010 and named after the US state of Tennessee, it has existed only a few atoms at a time, so its properties are predicted. Calculations suggest tennessine, and perhaps even astatine, would be more metallic than a typical halogen.