Halogen: Difference between revisions

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{{this|the chemical series}}
The '''halogens''' are the [[nonmetals|nonmetallic]] [[element|elements]] found in [[chemical group|Group 17]] of the [[periodic table|periodic table of the elements]]: [[fluorine]] ('''F'''), [[chlorine]] ('''Cl'''), [[bromine]] ('''Br'''), [[iodine]] ('''I'''), and [[astatine]] ('''At'''). The name "halogen" (Greek ''halos'', salt and ''-genes'', forming) is derived from their tendency to form a [[salt]] when reacted with a [[metal]].
[[Image:Halogens-3D-vdW.png|thumb|left|100px|The '''halogens''' exist as diatomic molecules in the [[gas]], [[liquid]] and [[solid]] [[Phase (matter)|phases]].  From top to bottom:<br>1. [[fluorine]], F<sub>2</sub><br>2. [[chlorine]], Cl<sub>2</sub><br>3. [[bromine]], Br<sub>2</sub><br>4. [[iodine]], I<sub>2</sub>]]
{| 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|ununseptium|#fcfece}}
|}


The '''halogens''' are a [[chemical series]]. They are the [[chemical element|elements]] in [[Periodic table group|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 [[France|French]] nomenclature based on erring adaptations of [[Greek language|Greek]] roots.
==Chemistry==


These elements are [[diatomic]] [[molecule]]s in their natural form. They require one more [[electron]] to fill their outer [[electron shell]]s, and so have a tendency to form a singly-charged [[Electric charge|negative]] [[ion]]. This negative ion is referred to as a ''halide'' ion; [[salt]]s containing these ions are known as [[halide]]s.
The valence shells of halogen atoms need only one more [[electron]] to form a completely filled shell of either eight or eighteen electrons. As such, the halogens have a strong tendency to form ions with a charge of -1 in order to complete their valence shells. They also do not exist as isolated atoms in their elemental forms; instead, they each share one electron with one other atom to form diatomic molecules.


Halogens are highly [[reactivity|reactive]], and as such can be harmful or lethal to [[Organism|biological organism]]s in sufficient quantities. Fluorine is the most reactive element in existence, even attacking glass, and forming compounds with the heavier [[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 (microbiology)|sterilization]]. Their [[Reaction (physics)|reactive]] properties are also put to use in [[Bleaching agent|bleaching]]. Chlorine is the active ingredient of most [[Cloth|fabric]] bleaches and is used in the production of most [[paper]] products.
A third consequence of the halogens' nearly filled valence shell is high [[reactivity]] towards cationic or low-ionization-potential species. Halogen [[anion|anions]] in the body can bind to [[potassium]], [[calcium]] and [[magnesium]] [[cation|cations]], rendering them useless. This is particularly true for fluoride ions, which can easily diffuse through the [[skin]] and enter the bloodstream.


Halide ions combined with single [[hydrogen]] [[atoms]] form the [[hydrohalic acid|''hydrohalic'' acid]]s (i.e., HF, HCl, HBr, HI), a series of particularly strong [[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]].)
[[hydrohalic acid|Hydrohalic acids]], diatomic compounds of one hydrogen atom and one halogen atom, are particularly strong owing to high halogen [[electron affinity]]. Care should be taken when handling these acids, particularly at high concentrations.


They react with each other to form [[interhalogen]] [[Chemical compound|compounds]]. Diatomic interhalogen compounds (BrF, ICl, ClF, etc.) bear strong superficial resemblance to the pure halogens.
The halogens are found in some [[hypervalent]] compounds (eg. SF<sub>6</sub>) and were instrumental in the formation of the first compounds of the [[noble gas|noble gases]] (eg. XEF<sub>4</sub>). They can also react with each other to form the so-called [[interhalogen|interhalogens]].


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 really essential for humans, although small amounts of fluoride can make tooth enamel resistant to decay.
==Properties==


They show a number of trends when moving down the group - for instance, decreasing electronegativity and reactivity, increasing melting and boiling point.
The halogens demonstrate several [[periodic trend|periodic trends]] very well. Their [[electronegativities|electronegativity]] and electron affinities decrease going down the group, and their sizes, melting and boiling points increase.


<table border=0 cellpadding=2 cellspacing=0 width="100%">
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   <td align="center">610 ?</td>
   <td align="center">610 ?</td>
   <td align="center">2.2</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>
  </tr>
</table>
</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.
{|style="text-align: center;" border="0" cellpadding="2"
|+ '''Explanation of above periodic table slice:'''
! bgcolor="{{element color/Halogens}}" | Halogens
| atomic number in <font color="{{element color/Gas}}">{{element color/Gas}}</font> are gases
| atomic number in <font color="{{element color/Liquid}}">{{element color/Liquid}}</font> are liquids
| atomic number in <font color="{{element color/Solid}}">{{element color/Solid}}</font> are solids
|-
| style="border:{{element frame/Primordial}};" | solid borders are [[primordial element]]s (older than the [[Earth]])
| style="border:{{element frame/Natural radio}};" | dashed borders are naturally [[radioactive decay|radioactive element]]s
| style="border:{{element frame/Synthetic}};" | dotted borders are [[radioactive decay|radioactive]], [[synthetic element]]s
| style="border:{{element frame/Undiscovered}};" | those without borders have not been discovered yet
|}
{{PeriodicTablesFooter}}
== References ==
# N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
==See also==
*[[pseudohalogen]]
[[Category:Halogens|*]]
[[Category:Periodic table]]
[[Category:Chemistry Workgroup]]


[[ar:هالوجين]]
Fluorine and chlorine are diatomic gases at room temperature, bromine is a liquid, and iodine and astatine are solids. Group 17 is the only group containing elements in all three [[chemical phase|phases]] at room temperature.
[[ast:Halóxenu]]
[[bg:Халоген]]
[[ca:Halogen]]
[[cs:Halogen]]
[[cy:Halogen]]
[[da:Halogen]]
[[de:Halogene]]
[[et:Halogeen]]
[[el:Αλογόνα]]
[[es:Halógeno]]
[[eo:Halogeno]]
[[fa:هالوژن]]
[[fo:Halogen]]
[[fr:Halogène]]
[[ko:할로젠]]
[[id:Halogen]]
[[is:Halógen]]
[[it:Alogeni]]
[[he:הלוגן]]
[[la:Halogenica]]
[[lv:Halogēni]]
[[lt:Halogenas]]
[[lmo:Alògen]]
[[hu:Halogén]]
[[ms:Halogen]]
[[nl:Halogeen]]
[[ja:第17族元素]]
[[no:Halogen]]
[[nn:Halogen]]
[[pl:Fluorowce]]
[[pt:Halogênio]]
[[ru:Галогены]]
[[simple:Halogen]]
[[sk:Halogén]]
[[sl:Halogen]]
[[sr:Халогени елементи]]
[[sh:Halogeni elementi]]
[[fi:Halogeeni]]
[[sv:Halogen]]
[[th:แฮโลเจน]]
[[vi:Halôgen]]
[[tr:Halojen]]
[[zh:卤素]]

Revision as of 23:22, 7 February 2007

The halogens are the nonmetallic elements found in Group 17 of the periodic table of the elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The name "halogen" (Greek halos, salt and -genes, forming) is derived from their tendency to form a salt when reacted with a metal.

Chemistry

The valence shells of halogen atoms need only one more electron to form a completely filled shell of either eight or eighteen electrons. As such, the halogens have a strong tendency to form ions with a charge of -1 in order to complete their valence shells. They also do not exist as isolated atoms in their elemental forms; instead, they each share one electron with one other atom to form diatomic molecules.

A third consequence of the halogens' nearly filled valence shell is high reactivity towards cationic or low-ionization-potential species. Halogen anions in the body can bind to potassium, calcium and magnesium cations, rendering them useless. This is particularly true for fluoride ions, which can easily diffuse through the skin and enter the bloodstream.

Hydrohalic acids, diatomic compounds of one hydrogen atom and one halogen atom, are particularly strong owing to high halogen electron affinity. Care should be taken when handling these acids, particularly at high concentrations.

The halogens are found in some hypervalent compounds (eg. SF6) and were instrumental in the formation of the first compounds of the noble gases (eg. XEF4). They can also react with each other to form the so-called interhalogens.

Properties

The halogens demonstrate several periodic trends very well. Their electronegativity and electron affinities decrease going down the group, and their sizes, melting and boiling points increase.

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

Fluorine and chlorine are diatomic gases at room temperature, bromine is a liquid, and iodine and astatine are solids. Group 17 is the only group containing elements in all three phases at room temperature.

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