Halogen

A halogen is an element in Group 17 of the periodic table, with seven valence electrons; the halogens are reactive non-metals that exist as diatomic molecules. Malay: halogen · Chinese: 卤素.

Fluorine, chlorine, bromine and iodine are the halogens you study. Because each atom has seven valence electrons, it needs to gain just one more to reach a stable octet, so a halogen reacts by gaining one electron to form an ion with a charge of −1, such as Cl⁻. In the free state the atoms pair up into diatomic molecules, Cl₂, Br₂, I₂, and the elements show a clear trend in colour and state down the group, from pale yellow-green chlorine gas, through red-brown liquid bromine, to dark grey solid iodine.

Example. Chlorine reacts with iron to form iron(III) chloride, and with sodium to form the halide salt sodium chloride. Down the group the reactivity falls: chlorine is more reactive than bromine, which is more reactive than iodine, because the larger atom lower down attracts an incoming electron less strongly, so it gains that electron less readily. This falling reactivity is the exact opposite of the pattern in Group 1, and being able to explain both trends from atomic size is a common higher-mark question.

Confusion to avoid. A halogen is the element (chlorine, Cl₂); a halide is the negative ion or salt it forms (chloride, Cl⁻). Keep the two words apart. Note too that the halogen reactivity trend is the reverse of the alkali metals: reactivity decreases down Group 17 but increases down Group 1, because one group gains electrons and the other loses them.

In the Periodic Table of Elements chapter the halogens are the clearest example of a reactive non-metal group, and they set up the displacement reactions in which a more reactive halogen pushes a less reactive one out of its salt.