Halogens are the elements in group 17 of the periodic table: fluorine, chlorine, bromine, iodine, astatine and tennessine. The name halogen means salt former, derived from Greek roots associated with salt and generation or formation, reflecting the ability of these elements to form salts with metals. The halogens are non metals or predominantly non metallic elements, with tennessine being a highly radioactive synthetic element whose properties are less well established.
They have seven valence electrons and therefore commonly tend to gain one electron to form halide ions with a minus 1 charge, or share one electron in covalent compounds. The group displays strong oxidising behaviour, especially among the lighter members, and the elemental halogens are generally reactive diatomic molecules represented as F2, Cl2, Br2, I2 and, for the heavier members under appropriate descriptions, At2. Their reactivity and physical properties show systematic trends down the group.
The electronic configuration of halogens explains their characteristic chemistry. Fluorine has seven valence electrons in the second shell, chlorine has seven in the third, and the heavier members similarly possess an outer ns2 np5 configuration. They require only one additional electron to complete the outer p subshell, making electron gain and electron sharing important aspects of their chemistry. Their high electronegativities allow them to attract bonding electrons strongly, with fluorine being the most electronegative element on the Pauling scale.
Atomic radius increases down the group because additional electron shells are added. Ionisation energy generally decreases down the group, while electronegativity and the ability to attract an additional electron generally decrease overall. These changes help explain why fluorine is an exceptionally powerful oxidising agent and why oxidising strength generally decreases down the group.
The halogens are highly reactive because their atoms can readily participate in reactions that produce more stable electron configurations. Fluorine is exceptionally reactive and can react with many elements and compounds, while chlorine is also strongly reactive and widely used as an oxidising and chlorinating agent. Bromine is less reactive than chlorine but remains chemically significant, while iodine is less strongly oxidising.
The reduction of halogen molecules to halide ions can be represented generally as X2 plus 2 electrons producing 2X minus. In this process, the halogen is reduced while another substance is oxidised, so elemental halogens commonly act as oxidising agents. Halide ions can themselves be oxidised back to elemental halogens by stronger oxidising agents. This relationship forms the basis of many displacement reactions and electrochemical processes.
Halogens react with metals to form ionic halides. Sodium reacts with chlorine to produce sodium chloride, magnesium reacts with chlorine to form magnesium chloride, and calcium reacts with fluorine to form calcium fluoride. The products contain positive metal ions and negative halide ions arranged in ionic lattices. Halogens also form covalent compounds with non metals, including hydrogen halides such as hydrogen chloride and hydrogen bromide.
Hydrogen fluoride is distinctive because its strong hydrogen bonding and other molecular properties make its behaviour different from the heavier hydrogen halides. Halogens also form numerous interhalogen compounds containing two or more different halogen elements, such as chlorine trifluoride and iodine monochloride. These compounds demonstrate that halogen chemistry extends well beyond simple metal salts.
The physical states of the halogens provide a particularly clear example of periodic trends. Fluorine and chlorine are gases under ordinary room conditions, bromine is a liquid, and iodine is a solid. Astatine is a highly radioactive solid, while tennessine has been produced only in extremely small quantities and its bulk physical properties have not been directly observed in the ordinary sense. Down the group, atomic size and polarizability increase, strengthening London dispersion forces between diatomic molecules.
This causes melting and boiling points to increase sufficiently for the physical state to change from gas to liquid to solid under comparable conditions. The colours also become generally deeper down the group. Fluorine is very pale yellow, chlorine is yellow green, bromine is reddish brown and iodine is dark grey or black as a solid and produces a violet vapour when heated.
Halogens have major practical applications, although their elemental forms can be hazardous because of their high reactivity. Chlorine is widely used to disinfect drinking water and swimming pools and is important in the manufacture of numerous chemicals and polymers. Fluorine chemistry is essential to many materials, pharmaceuticals and industrial compounds, although elemental fluorine is extremely reactive and difficult to handle.
Bromine compounds have applications in specialised materials, chemical synthesis and flame retardant chemistry, while iodine is important in medicine, analytical chemistry and biological nutrition. Iodide is required in small amounts for thyroid hormone synthesis, and iodine based compounds have been used in antiseptics and medical imaging. The usefulness of a halogen depends strongly on its chemical form, because the properties of elemental halogens can differ radically from those of their stable salts and organic compounds.
Halogen chemistry is central to analytical chemistry and periodic trend demonstrations. A more reactive halogen can displace a less reactive halide from solution. For example, chlorine can oxidise bromide ions to bromine and iodide ions to iodine, whereas bromine can oxidise iodide ions but generally cannot oxidise chloride ions under ordinary aqueous conditions. These reactions reveal the relative oxidising strengths of the elemental halogens.
Silver nitrate is also commonly used in analytical chemistry to test for halide ions, producing characteristic precipitates whose solubilities differ. Fluoride, chloride, bromide and iodide therefore have distinct analytical behaviours despite sharing the same group classification. The chemistry also illustrates how periodic trends operate in actual reactions rather than existing merely as theoretical patterns in the periodic table.
Halogens should be distinguished from halide ions and from halogen containing compounds. A halogen is the elemental group 17 substance, whereas a halide is the negative ion or chemical species derived from a halogen after gaining or otherwise possessing one additional electron, such as chloride or bromide. Chlorine gas, for example, is a reactive oxidising substance, whereas chloride ions in sodium chloride are comparatively stable and are essential electrolytes in biological systems.
Another misconception is that all halogens are equally reactive. Reactivity varies substantially down the group, and fluorine is far more reactive than iodine in many contexts. Astatine and tennessine also cannot simply be treated as ordinary versions of fluorine or chlorine because radioactivity, scarcity and heavy atom effects strongly limit experimental knowledge. Halogens are therefore best understood as a chemically related family whose shared seven valence electron configuration produces broad similarities while atomic size, electron structure and environmental conditions generate substantial differences.
