8.7: Halogens

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.

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.

Halogens are a group of five nonmetallic elements – fluorine, chlorine, bromine, iodine, and astatine – belonging to Group 17. Elemental halogens exist as diatomic molecules. 

At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Less is known about the properties of the fifth element, astatine, which is a rare and highly radioactive solid. 

The physical properties of halogens, such as elemental color, vary moving down the group. Atomic radii increase with increasing atomic number, and the melting point and boiling point show an upward trend from fluorine to iodine. In contrast, periodic properties like electronegativity, reactivity, and electron affinity decrease down the group.        

Electron configurations of halogens indicate that they have seven valence electrons and are one electron short of attaining their nearest noble gas configuration. Therefore, halogens exhibit high electronegativities with a strong tendency to gain one additional electron to complete their octet, making them highly reactive and powerful oxidizing agents compared to other groups.

Fluorine is the most potent oxidizing agent amongst halogens and iodine the least.

Halogens also exhibit highly negative electron affinities, which is the change in energy when an electron is gained by a neutral atom. It is expressed in kJ/mol, and the negative value indicates the exothermic nature of the reaction. 

Halogens react with most metals to yield metal halides. For example, chlorine and sodium react to produce sodium chloride, which is an ionic compound.

Halogens react with hydrogen to form polar covalent compounds called hydrogen halides, which often dissolve in water to form hydrohalic acids. 

Halogens also react with water. The reaction of fluorine with water is highly exothermic, producing hydrogen fluoride — a strong acid — and oxygen. Chlorine, however, reacts slower, producing hydrochloric acid and hypochlorous acid — a strong oxidizing agent. 

Additionally, halogens can react with each other, producing binary, covalent compounds called interhalogen compounds. For instance, bromine reacts with fluorine forming bromine trifluoride.