Group 17

Properties These elements all have seven valence electrons. This group is the first one to consist of entirely non-metals. They exist as diatomic molecules in their natural state and have a progressive variation of physical properties (see table below). Fluorine and chlorine exist as gases at room temperature, bromine as a liguid, and iodine as a solid. They require one more electron to fill their outer electron shells, and so have a tendency to gain one electron to form a singly-charged negative ions. These negative ions are referred to as halide ions, and salts containing these ions are known as halides. Halogens are highly reactive, and as such can be harmful or lethal to biological organisms in sufficient quantities. Fluorine is the most reactive and the reactivity declines as we go down the group. Chlorine and iodine are both used as disinfectants. In their elemental state the halogens are oxidizing agents and are used in bleaches. Chlorine is the active ingredient of most fabric bleaches and is used in the production of most paper products. The oxides and hydrides, like those of most non-metals, of the halogens are acidic. Halide ions combined with single hydrogen atoms form the hydrohalic acids (i.e., HF, HCl, HBr, HI), a series of particularly strong 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 radioactive alpha decay.) They can react with each other to form interhalogen compounds, and can combine with oxygen in polyatomic oxoanions. Diatomic interhalogen compounds (BrF, ICl, ClF, etc.) bear strong superficial resemblance to the pure halogens. Many synthetic organic compounds, 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 human beings. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter Gamma-aminobutyric acid (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 really essential for humans, although small amounts of fluoride can make tooth enamel resistant to decay. The term halogen was coined to mean elements which produce salt in union with a metal. It comes from eighteenth century scientific French nomenclature based on erring adaptations of Greek roots. Table of trends in melting point, boiling point, and electronegativity. Atomic Mass (u) Fluorine 18.998 Chlorine 35.453 Bromine 79.904 Iodine 126.904 Astatine (210) Ununseptium (291)* Halogen

Melting Point (º Boiling Point (º C) C) −219.62 −188/12 −101.15 −34.04 −7.35 58.86 113.70 202.25 302 337 ? * *

Electronegativity (Pauling) 3.98 3.16 2.96 2.66 2.2 *

* Ununseptium has not yet been discovered; values are either unknown if no value appears, or are estimates based on other similar elements