Definition · Plain-language
Halogens
The halogens are the reactive non-metals in Group 17 of the periodic table, each one electron short of a full outer shell.
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The reactive non-metals of Group 17
The halogens form Group 17, near the right of the periodic table: fluorine, chlorine, bromine, iodine and the rare, radioactive astatine. The name means "salt-forming", because they react with metals to make salts such as sodium chloride. They are notable for spanning three states of matter at room temperature: fluorine and chlorine are gases, bromine is one of only two liquid elements, and iodine is a solid. As pure elements they exist as diatomic molecules — F₂, Cl₂, Br₂, I₂ — and many are coloured and toxic.
One electron short of stability
Each halogen atom has seven electrons in its outer shell, just one short of the stable, full arrangement of eight that the noble gases enjoy. This makes them eager to gain a single electron, forming a negative ion with a single charge, such as the chloride ion (Cl⁻). That same eagerness lets them form covalent bonds by sharing an electron pair. Being one electron short of a full shell is the key to all their chemistry and explains why they are among the most reactive non-metals.
Trends and everyday importance
Reactivity decreases down the group — the opposite trend to the alkali metals — because the outer shell lies further from the nucleus in the heavier halogens, so an incoming electron is attracted less strongly. Fluorine is therefore the most reactive. The halogens and their compounds are woven through daily life: chlorine disinfects drinking water and swimming pools, fluoride compounds help protect teeth, iodine is an essential nutrient and antiseptic, and many plastics and refrigerants contain halogens.
Key facts
At a glance
- Definition: the reactive non-metals of Group 17 of the periodic table
- Members: fluorine, chlorine, bromine, iodine, astatine
- Outer electrons: seven — one short of a full shell
- Ion formed: a single negative charge (e.g. Cl⁻)
- Reactivity: decreases down the group
- As elements: diatomic molecules (F₂, Cl₂, Br₂, I₂)
Common misconceptions
What people often get wrong
Often heard: All the halogens are gases.
Actually: They span three states at room temperature: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. This variety within a single group is one of their distinctive features.
Often heard: Iodine is the most reactive halogen because it is the largest.
Actually: Reactivity decreases down the group, so fluorine is the most reactive and iodine much less so. The larger outer shell of the heavier halogens attracts an incoming electron less strongly.
Often heard: Halogens lose electrons like metals do.
Actually: Halogens are non-metals that gain or share electrons, not lose them. Each is one electron short of a full shell, so it tends to take on an electron, forming a negative ion.
Going deeper
