Halogen



The halogens or halogen elements are a series of nonmetal elements from Group 17 (old-style: VII or VIIA; Group 7 IUPAC Style) of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At. The undiscovered element 117, temporarily named ununseptium, may also be considered a halogen.

The group of halogens is the only group which contains elements in all three familiar states of matter at standard temperature and pressure.

Abundance
Owing to their high reactivity, the halogens are found in the environment only in compounds or as ions. Halide ions and oxoanions such as IO3− can be found in many minerals and in seawater. Halogenated organic compounds can also be found as natural products in living organisms. In their elemental forms, the halogens exist as diatomic molecules, but these only have a fleeting existence in nature and are much more common in the laboratory and in industry. At room temperature and pressure, fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids; Group 17 is therefore the only periodic table group exhibiting all three states of matter at room temperature.

Etymology
The term halogen originates from 18th century scientific French nomenclature based on adaptations of Greek roots: hals (sea) or halas (salt), and gen- (to generate) — referring to elements which produce a salt in union with a metal.

Properties
The halogens show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point. * Ununseptium has not yet been discovered; values are either unknown if no value appears, or are estimates based on other similar chemicals.

Reactivity
Halogens are highly reactive, and as such can be harmful or lethal to biological organisms in sufficient quantities. This high reactivity is due to their atoms being one electron short of a full outer shell of eight electrons. They can gain this electron by reacting with atoms of other elements. Fluorine is the most reactive element in existence, attacking such inert materials as glass, and forming compounds with the heavier noble gases. It is a corrosive and highly toxic gas. The reactivity of fluorine is such that, if used or stored in laboratory glassware, it can react with glass in the presence of small amounts of water to form SiF4. Thus fluorine must be handled with substances such as Teflon, extremely dry glass, or metals such as copper or steel which form a protective layer of fluoride on their surface.

Both chlorine and bromine are used as disinfectants for drinking water, swimming pools, fresh wounds, dishes, and surfaces. They kill bacteria and other potentially harmful microorganisms through a process known as sterilization. Their reactivity is also put to use in bleaching. Sodium hypochlorite, which is produced from chlorine, is the active ingredient of most fabric bleaches and chlorine-derived bleaches are used in the production of some paper products.

Hydrogen halides
The halogens all form binary compounds with hydrogen, the hydrogen halides, HX (HF, HCl, HBr, HI), a series of particularly strong acids. When in aqueous solution, the hydrogen halides are known as hydrohalic acids. HAt, or "hydrastatic acid", should also qualify, but it is not typically included in discussions of hydrohalic acid due to astatine's extreme instability toward alpha decay.

Interhalogen compounds
The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds (e.g. BrF, ICl, ClF) bear resemblance to the pure halogens in some respects. The properties and behaviour of a diatomic interhalogen compound tend to be intermediate between those of its parent halogens. Some properties, however, are found in neither parent halogen − Cl2 and I2 are soluble in CCl4 but ICl is not, since it is a polar molecule due to the relatively large electronegativity difference between I and Cl.

Organohalogen compounds
Many synthetic organic compounds such as plastic polymers, and a few natural ones, contain halogen atoms; these are known as halogenated compounds or organic halides. Chlorine is by far the most abundant of the halogens, and the only one needed in relatively large amounts (as chloride ions) by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA and are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones such as thyroxine. On the other hand, neither fluorine nor bromine are believed to be essential for humans, although small amounts of fluoride can make tooth enamel resistant to decay.

Drug discovery
In drug discovery, the incorporation of halogen atoms into a lead drug candidate results in analogues that are more lipophilic and less water soluble. Consequently, halogen atoms are used to improve penetration through lipid membranes. However, there is an undesirable tendency for halogenated drugs to accumulate in lipid tissue.

The chemical reactivity of halogen atoms depends on both their point of attachment to the lead and the nature of the halogen. Aromatic halogen groups are far less reactive than aliphatic halogen groups, which can exhibit considerable chemical reactivity. For aliphatic carbon-halogen bonds the C-F bond is the strongest and usually less chemically reactive than aliphatic C-H bonds. The other aliphatic-halogen bonds are weaker, their reactivity increasing down the periodic table. They are usually more chemically reactive than aliphatic C-H bonds. Consequently, the most popular halogen substitutions are the less reactive aromatic fluorine and chlorine groups.