Halogens

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.