View source for Halogen



{{otheruses1|the chemical series}}
{| align="right" style="margin:0 0 1em 1em;"
! [[Periodic table group|Group]]
! [[Group 17 element|17]]
|-
! [[Periodic table period|Period]]
|-
! [[Period 2 element|2]]
| {{element cell| 9|Fluorine|F| |Gas|Halogens|Primordial}}
|-
! [[Period 3 element|3]]
| {{element cell|17|Chlorine|Cl| |Gas|Halogens|Primordial}}
|-
! [[Period 4 element|4]]
| {{element cell|35|Bromine|Br| |Liquid|Halogens|Primordial}}
|-
! [[Period 5 element|5]]
| {{element cell|53|Iodine|I| |Solid|Halogens|Primordial}}
|-
! [[Period 6 element|6]]
| {{element cell|85|Astatine|At| |Solid|Halogens|Natural radio}}
|-
! [[Period 7 element|7]] 
| {{element cell|117|Ununseptium|Uus| |Solid|Halogens|Undiscovered}}
|-
|}

[[Image:Halogene.jpg|thumb|320px|right]]

The '''halogens''' or '''halogen elements''' are a [[chemical series|series]] of [[nonmetal]] [[chemical element|elements]] from [[Periodic table group|Group 17]] (old-style: VII or VIIA; Group 7 [[International Union of Pure and Applied Chemistry|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 [[chemical compound|compounds]] or as [[ion]]s. [[Halide]] ions and [[oxyanion|oxoanions]] such as [[iodate|IO<sub>3</sub><sup>−</sup>]] can be found in many minerals and in seawater. [[organic halide|Halogenated organic compounds]] can also be found as natural products in living organisms. In their elemental forms, the halogens exist as [[diatomic molecule]]s, 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 [[state of matter|states of matter]] at room temperature.

==Etymology==
The term ''halogen'' originates from [[18th century]] scientific [[France|French]] nomenclature based on adaptations of [[Greek language|Greek]] roots: ''hals'' (sea) or ''halas'' (salt), and ''gen-'' (to ''gene''rate) — 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 point|melting]] and [[boiling point]]. 
<table border=0 cellpadding=2 cellspacing=0 width="100%">
 <tr>
 <td bgcolor="#CCCCFF">'''Halogen'''</td>
 <td bgcolor="#CCCCFF" align="center">'''Standard Atomic Weight ([[unified atomic mass unit|u]])'''</td>
 <td bgcolor="#CCCCFF" align="center">'''Melting Point ([[kelvin|K]])'''</td>
 <td bgcolor="#CCCCFF" align="center">'''Boiling Point ([[kelvin|K]])'''</td>
 <td bgcolor="#CCCCFF" align="center">'''Electronegativity ([[Pauling scale|Pauling]])'''</td>
 </tr>

 <tr>
 <td>[[Fluorine]]</td>
 <td align="center">18.998</td>
 <td align="center">53.53</td>
 <td align="center">85.03</td>
 <td align="center">3.98</td>
 </tr>
 <tr>
 <td bgcolor="#EEEEEE">[[Chlorine]]</td>
 <td bgcolor="#EEEEEE" align="center">35.453</td>
 <td bgcolor="#EEEEEE" align="center">171.6</td>
 <td bgcolor="#EEEEEE" align="center">239.11</td>
 <td bgcolor="#EEEEEE" align="center">3.16</td>
 </tr>
 <tr>
 <td>[[Bromine]]</td>
 <td align="center">79.904</td>
 <td align="center">265.8</td>
 <td align="center">332.0</td>
 <td align="center">2.96</td>
 </tr>
 <tr>
 <td bgcolor="#EEEEEE">[[Iodine]]</td>
 <td bgcolor="#EEEEEE" align="center">126.904</td>
 <td bgcolor="#EEEEEE" align="center">386.85</td>
 <td bgcolor="#EEEEEE" align="center">457.4</td>
 <td bgcolor="#EEEEEE" align="center">2.66</td>
 </tr>
 <tr>
 <td>[[Astatine]]</td>
 <td align="center">(210)</td>
 <td align="center">575</td>
 <td align="center">610 ?</td>
 <td align="center">2.2</td>
 </tr>
 <tr>
 <td>[[Ununseptium]]</td>
 <td align="center">(291)*</td>
 <td align="center">*</td>
 <td align="center">*</td>
 <td align="center">*</td>
 </tr>
</table>
<nowiki>*</nowiki> 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==
{| class="wikitable" style="margin: 1em auto 1em auto"
! halogen || molecule || structure || model || ''d''(X−X) / pm<br />(gas phase) || ''d''(X−X) / pm<br />(solid phase)
|-
| <center>fluorine</center> || <center>F<sub>2</sub></center> || <center>[[Image:Difluorine-2D-dimensions.png|45px]]</center> || <center>[[Image:Fluorine-3D-vdW.png|45px]]</center> || <center>143</center> || <center>149</center>
|-
| <center>chlorine</center> || <center>Cl<sub>2</sub></center> || <center>[[Image:Dichlorine-2D-dimensions.png|70px]]</center> || <center>[[Image:Chlorine-3D-vdW.png|63px]]</center> || <center>199</center> || <center>198</center>
|-
| <center>bromine</center> || <center>Br<sub>2</sub></center> || <center>[[Image:Dibromine-2D-dimensions.png|80px]]</center> || <center>[[Image:Bromine-3D-vdW.png|72px]]</center> || <center>228</center> || <center>227</center>
|-
| <center>iodine</center> || <center>I<sub>2</sub></center> || <center>[[Image:Diiodine-2D-dimensions.png|70px]]</center> || <center>[[Image:Iodine-3D-vdW.png|84px]]</center> || <center>266</center> || <center>272</center>
|}

==Chemistry==
===Reactivity===
Halogens are highly [[reactivity|reactive]], and as such can be harmful or lethal to [[Organism|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 SiF<sub>4</sub>. Thus fluorine must be handled with substances such as [[polytetrafluoroethylene|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 (microbiology)|sterilization]]. Their reactivity is also put to use in [[Bleaching agent|bleaching]]. [[Sodium hypochlorite]], which is produced from chlorine, is the active ingredient of most [[Cloth|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 halide]]s, HX ([[hydrogen fluoride|HF]], [[hydrogen chloride|HCl]], [[hydrogen bromide|HBr]], [[hydrogen iodide|HI]]), a series of particularly strong [[acid]]s. When in aqueous solution, the hydrogen halides are known as [[hydrohalic acid]]s. 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|Interhalogen}}
The halogens react with each other to form interhalogen compounds. Diatomic interhalogen compounds (e.g. [[bromine monofluoride|BrF]], [[iodine monochloride|ICl]], [[chlorine monofluoride|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 − Cl<sub>2</sub> and I<sub>2</sub> are soluble in [[carbon tetrachloride|CCl<sub>4</sub>]] but ICl is not, since it is a [[chemical polarity|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 halide]]s. 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 membrane]]s. 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 ==
# N. N. Greenwood, A. Earnshaw, ''[http://www.knovel.com/knovel2/Toc.jsp?BookID=402 Chemistry of the Elements]'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
# G. Thomas, ''Medicinal Chemistry an Introduction'', John Wiley & Sons, West Sussex, UK, 2000.


{|style="text-align: center;" border="0" cellpadding="2" 
|+ '''Explanation of above periodic table slice:'''
! bgcolor="{{element color/Halogens}}" | Halogens
| Atomic numbers in <font color="{{element color/Gas}}">{{element color/Gas}}</font> are gases
| Atomic numbers in <font color="{{element color/Liquid}}">{{element color/Liquid}}</font> are liquids
| Atomic numbers in <font color="{{element color/Solid}}">{{element color/Solid}}</font> are solids
|-
| style="border:{{element frame/Primordial}};" | Solid borders indicate [[primordial element]]s (older than the [[Earth]])
| style="border:{{element frame/Natural radio}};" | Dashed borders indicate [[radioactive decay|radioactive]] natural elements
| style="border:{{element frame/Synthetic}};" | Dotted borders indicate radioactive [[synthetic element]]s
| style="border:{{element frame/Undiscovered}};" | No borders indicates undiscovered elements
|}


{{PeriodicTablesFooter}}

[[Category:Halogens| ]]
[[Category:Periodic table]]

[[ar:هالوجين]]
[[ast:Halóxenu]]
[[bn:হ্যালোজেন]]
[[bs:Halogeni elementi]]
[[bg:Халоген]]
[[ca:Halogen]]
[[cs:Halogen]]
[[cy:Halogen]]
[[da:Halogener]]
[[de:Halogene]]
[[et:Halogeenid]]
[[el:Αλογόνα]]
[[es:Halógeno]]
[[eo:Halogeno]]
[[eu:Halogeno]]
[[fa:هالوژن]]
[[fo:Halogen]]
[[fr:Halogène]]
[[ko:할로젠]]
[[id:Halogen]]
[[is:Halógen]]
[[it:Alogeno]]
[[he:הלוגן]]
[[la:Halogenica]]
[[lv:Halogēni]]
[[lt:Halogenas]]
[[jbo:klirylei]]
[[lmo:Alògen]]
[[hu:Halogének]]
[[ms:Halogen]]
[[nl:Halogeen]]
[[ja:第17族元素]]
[[no:Halogen]]
[[nn:Halogen]]
[[nds:Halogen]]
[[pl:Fluorowce]]
[[pt:Halogênio]]
[[qu:Kachichaq]]
[[ru:Галогены]]
[[sq:Halogjeni]]
[[simple:Halogen]]
[[sk:Halogény]]
[[sl:Halogeni]]
[[sr:Халогени елементи]]
[[sh:Halogeni elementi]]
[[fi:Halogeeni]]
[[sv:Halogen]]
[[th:แฮโลเจน]]
[[vi:Halogen]]
[[tr:Halojen]]
[[uk:Галогени]]
[[zh-yue:鹵素]]
[[zh:卤素]]

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