Halogen
This article discusses the group of chemical elements in the periodic table. For the light bulb, see the halogen lamp.The
halogens are a
chemical series. They are the
elements in
Group 17 (old-style: VII or VIIA) of the
periodic table:
fluorine (
F),
chlorine (
Cl),
bromine (
Br),
iodine (
I),
astatine (
At) and the as yet undiscovered
ununseptium (
Uus). The term halogen was coined to mean elements which produce
salt in union with a
metal. It comes from
18th century scientific
French nomenclature based on erring adaptations of
Greek roots.
These elements are
diatomic molecules in their natural form. They require one more
electron to fill their outer
electron shells, and so have a tendency to form a singly-charged
negative ion. This negative ion is referred to as a
halide ion;
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 element in existence, even attacking glass, and forming compounds with the
noble gases. It is a corrosive, highly toxic gas. Chlorine and iodine are both used as
disinfectants for such things as
drinking water, swimming pools, fresh wounds, dishes, and surfaces. They kill
bacteria and other potentially harmful
microorganisms, a process known as
sterilization. Their
reactive properties are also put to use in
bleaching. Chlorine is the active ingredient of most
fabric bleaches and is used in the production of most
paper products.
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
alpha decay.)
They react with each other to form
interhalogen compounds. Diatomic interhalogen compounds (BrF, ICl, ClF, etc.) bear strong superficial resemblance to the pure halogens.
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 really essential for humans, although small amounts of fluoride can make tooth enamel resistant to decay.
They show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point.
| Halogen | Atomic Mass (u) | Melting Point (K) | Boiling Point (K) | Electronegativity (Pauling) |
| Fluorine | 18.998 | 53.53 | 85.03 | 3.98 |
| Chlorine | 35.453 | 171.6 | 239.11 | 3.16 |
| Bromine | 79.904 | 265.8 | 332.0 | 2.96 |
| Iodine | 126.904 | 386.85 | 457.4 | 2.66 |
| Astatine | (210) | 575 | 610 ? | 2.2 |
| Ununseptium | (291)* | * | * | * |
* Ununseptium has not yet been discovered; values are either unknown if no value appears, or are estimates based on other similar chemicals.
*
pseudohalogen
