Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. Astatine is a highly unstable radioactive element, so currently, most of its properties are unknown due to its short half-life. Tennessine is a synthetic element also predicted to be in this group.

The halogens are not found as single atoms but exist as diatomic molecules. The atomic radius increases from fluorine to iodine. The valence shell electron configuration of halogens is ns²np⁵, and they have a tendency to accept an electron to achieve the noble gas configuration. The various properties of halogens are listed in table 1.

Table 1: Properties of the Halogens.

Element	Electron Configuration	Atomic Radius (pm)	IE₁(kJ/mol)	EA (kJ/mol)	Density at 25 °C	Melting Point (°C)
F	[He] 2s²2p⁵	71	1680	-328	1.70 g/L	-219
Cl	[Ne] 3s²3p⁵	99	1250	-348	3.12 g/L	-101
Br	[Ar] 4s²4p⁵	114	1140	-324	3.19 g/cm³	-7
I	[Kr] 5s²5p⁵	133	1010	-295	3.96 g/cm³	114

The electron affinities of halogens have large negative values; thus, the halogens are powerful oxidizing agents. Fluorine is the most powerful oxidizing agent of the group with the most negative electron affinity. The halogens react with metals to gain the electron and produce corresponding metal halide. For example, chlorine reacts with iron to give iron chloride and with sodium to give sodium chloride. The halogens react with hydrogen to form hydrogen halides, which dissolve in water to produce hydrohalic acids. Hydrofluoric acid is the weakest haloacid. Fluorine reacts vigorously with water. The reaction is highly exothermic and produces oxygen. The reaction of chlorine is less vigorous with water. The hypochlorous acid produced in this reaction is a strong oxidizing agent.

The halogens react with each other to form interhalogen compounds, many of which are binary compounds. The general molecular formula for interhalogen compounds is AB_n, where A is the halogen with higher atomic number and is an odd number. Few examples of interhalogen compounds are ICl, IBr, BrF, BrCl, ClF. The halogens react with carbon to form commercially important compounds. For example, ethylene reacts with fluorine to give perfluoroethylene.

This text is adapted from Openstax, Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.