Halogen

Halogen
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This article is about the chemical series. For other uses, see Halogen (disambiguation).
Group 17
Period
2 9
F
3 17
Cl
4 35
Br
5 53
I
6 85
At

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, astatine, At.

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

Contents [hide]
1 Abundance
2 Etymology
3 Properties
4 Diatomic halogen molecules
5 Chemistry
5.1 Reactivity
5.2 Hydrogen halides
5.3 Interhalogen compounds
6 Organohalogen compounds
7 Drug discovery
8 See also
9 References



[edit] 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.


[edit] 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.


[edit] 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.


[edit] 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


[edit] Chemistry

[edit] Reactivity
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, 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 flourine 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.


[edit] 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.


[edit] 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.


[edit] 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.


[edit] 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.


[edit] See also
Pseudohalogen

[edit] References
N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
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




[hide]v • d • ePeriodic 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