halogen

 

(hăl'əjĕn) [Gr.,=salt-bearing], any of the chemically active elements found in Group 17 of the periodic table; the name applies especially to fluorine (symbol F), chlorine (Cl), bromine (Br), and iodine (I). Astatine (At), formerly known as alabamine, is a radioactive element also classed as a halogen; its most stable isotope (which does not occur in nature) has a half-life of less than 8¹/₂ hr. The chemical and physical properties of astatine are not well known; it is believed to resemble iodine. The halogens are the best-defined family of chemical elements. Chemically they closely resemble one another; they are nonmetallic and form monovalent negative ions. They also exhibit an almost perfect gradation of physical properties. Fluorine, a pale yellow gas, is the least dense and chemically the most active, displacing the other halogens from their compounds and even displacing oxygen from water. Chlorine, a yellow-green gas, is more dense and less reactive than fluorine. Bromine is a dark red liquid. Iodine is a grayish black solid and is the least chemically active of the four; however, among the nonmetals only oxygen is more reactive than iodine. Pure halogens exist as diatomic molecules, e.g., Cl₂; they form interhalogen compounds, i.e., compounds between two halogens. The halogens form numerous compounds with other elements. With hydrogen they form hydrogen halides, whose water solutions are called hydrohalic acids, e.g., the water solution of hydrogen chloride is called hydrochloric acid. They form numerous metal halides, or salts, e.g., sodium chloride, common table salt. They also form halocarbons, compounds with carbon and often other elements such as hydrogen and oxygen. Chloroform, iodoform, and carbon tetrachloride are halocarbons. Some other halogen compounds are calomel (mercurous chloride), fluorite, sal ammoniac (ammonium chloride), corrosive sublimate (mercuric chloride), and chlorine bleaches.

Group	17
Period
2	9 F
3	17 Cl
4	35 Br
5	53 I
6	85 At
7	117 Uus

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 7 is therefore the only periodic table group exhibiting all three states of matter.

Etymology

The term halogen originates from 18th century scientific French nomenclature based on erring adaptations of Greek roots; the Greek word halos meaning "salt", and genes meaning "production" — 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.

Halogen	Standard Atomic Weight (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.

Diatomic halogen molecules

halogen	molecule	structure	model	d(X−X) / pm (gas phase)	d(X−X) / pm (solid phase)
fluorine	F2			143	149
chlorine	Cl2			199	198
bromine	Br2			228	227
iodine	I2			266	272

Chemistry

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. Chlorine is the active ingredient of most fabric bleaches and is used in the production of most 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

Main article: Interhalogen

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.

See also

• Pseudohalogen

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. G. Thomas, Medicinal Chemistry an Introduction, John Wiley & Sons, West Sussex, UK, 2000.

Explanation of above periodic table slice:

Halogens	Atomic numbers in red are gases	Atomic numbers in green are liquids	Atomic numbers in black are solids
Solid borders indicate primordial elements (older than the Earth)	Dashed borders indicate radioactive natural elements	Dotted borders indicate radioactive synthetic elements	No borders indicates undiscovered elements

Periodic tables
Layouts	Standard · Vertical · Full names · Names and atomic masses · Text for last · Huge table · Metals and nonmetals · Blocks · Valences · Inline f-block · 218 elements · Electron configurations · Atomic masses · Electronegativities · Alternatives
Lists of elements	Name · Atomic symbol · Atomic number · Boiling point · Melting point · Density · Atomic mass
Groups	1 ·  2 ·  3 ·  4 ·  5 ·  6 ·  7 ·  8 ·  9 ·  10 ·  11 ·  12 ·  13 ·  14 ·  15 ·  16 ·  17 ·  18
Periods:	1 ·  2 ·  3 ·  4 ·  5 ·  6 ·  7 ·  8
Series	Alkalis ·  Alkaline earths ·  Lanthanides ·  Actinides ·  Transition metals · Poor metals ·  Metalloids ·  Nonmetals ·  Halogens ·  Noble gases
Blocks	s-block ·  p-block ·  d-block ·  f-block ·  g-block

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