Halogen

Abundance

Owing to their high 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 metal.

Properties

The halogens show a number of trends when moving down the group - for instance, decreasing 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 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 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 (alpha decay.

Interhalogen compounds

Main article: Interhalogen

The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds (e.g. BrF, electronegativity difference between I and Cl.

Organohalogen compounds

Many synthetic 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 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
• Halogen bond

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