Editing Gallium (section)

==Physical properties==
[[File:Gallium kristallisiert.JPG|thumb|left|Crystallization of gallium from the melt]]
Elemental gallium is not found in nature, but it is easily obtained by [[smelting]]. Very pure gallium is a silvery blue metal that fractures [[conchoidal fracture|conchoidally]] like [[glass]]. Gallium's volume expands by 3.10% when it changes from a liquid to a solid so care must be taken when storing it in containers that may rupture when it changes state. Gallium shares the higher-density liquid state with a short list of other materials that includes [[properties of water|water]], [[silicon]], [[germanium]], [[bismuth]], and [[plutonium]].<ref name="GreenwoodEarnshaw2nd">{{Greenwood&Earnshaw2nd}}</ref>{{rp|222}}
[[File:Liquid gallium pouring.png|thumb|Liquid gallium at {{convert|86|F|}}]]
Gallium forms alloys with most metals. It readily diffuses into cracks or [[grain boundary|grain boundaries]] of some metals such as aluminium, [[aluminium]]–[[zinc]] [[alloy]]s<ref>{{cite journal|title= Grain boundary imaging, gallium diffusion and the fracture behavior of Al–Zn Alloy – An in situ study |author= Tsai, W. L|journal= Nuclear Instruments and Methods in Physics Research Section B |date= 2003 |volume= 199 |pages= 457–463 |doi= 10.1016/S0168-583X(02)01533-1|bibcode= 2003NIMPB.199..457T|last2= Hwu|first2= Y.|last3= Chen|first3= C. H.|last4= Chang|first4= L. W.|last5= Je|first5= J. H.|last6= Lin|first6= H. M.|last7= Margaritondo|first7= G.}}</ref> and [[steel]],<ref>{{cite web|url=https://apps.dtic.mil/sti/citations/ADA365497 |title= Liquid Metal Embrittlement of ASTM A723 Gun Steel by Indium and Gallium|author= Vigilante, G. N.|author2= Trolano, E.|author3= Mossey, C.|publisher= Defense Technical Information Center |date=June 1999|access-date=7 July 2009}}</ref> causing extreme loss of strength and ductility called [[liquid metal embrittlement]].

The [[melting point]] of gallium, at 302.9146&nbsp;K (29.7646&nbsp;°C, 85.5763&nbsp;°F), is just above room temperature, and is approximately the same as the average summer daytime temperatures in Earth's mid-latitudes. This melting point (mp) is one of the formal temperature reference points in the [[International Temperature Scale of 1990]] (ITS-90) established by the [[International Bureau of Weights and Measures]] (BIPM).<ref>{{cite journal |url= http://www.bipm.org/utils/common/pdf/its-90/ITS-90_metrologia.pdf |archive-url=https://web.archive.org/web/20070618131554/http://www.bipm.org/utils/common/pdf/its-90/ITS-90_metrologia.pdf |archive-date=18 June 2007 |url-status=live |title= The International Temperature Scale of 1990 (ITS-90) |last= Preston–Thomas |first= H. |journal= Metrologia |volume= 27 |issue= 1 |pages= 3–10 |date= 1990 |doi= 10.1088/0026-1394/27/1/002 |bibcode= 1990Metro..27....3P |s2cid= 250785635 }}</ref><ref>{{Cite web |url=http://www.bipm.org/en/publications/its-90.html |title=ITS-90 documents at Bureau International de Poids et Mesures}}</ref><ref>{{cite news |url=http://www.cstl.nist.gov/div836/836.05/papers/magnum90ITS90guide.pdf |archive-url=https://web.archive.org/web/20030704215942/http://www.cstl.nist.gov/div836/836.05/papers/magnum90ITS90guide.pdf |url-status=dead |archive-date=4 July 2003 |title=Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90) |last1=Magnum |first1=B. W. |last2=Furukawa |first2=G. T. |publisher=National Institute of Standards and Technology |id=NIST TN 1265 |date=August 1990 }}</ref> The [[triple point]] of gallium, 302.9166&nbsp;K (29.7666&nbsp;°C, 85.5799&nbsp;°F), is used by the US [[National Institute of Standards and Technology]] (NIST) in preference to the melting point.<ref>{{cite journal|access-date=30 October 2016 |title=NIST realization of the gallium triple point |last=Strouse |first=Gregory F. |journal=Proc. TEMPMEKO |volume=1999 |issue=1 |year=1999|pages=147–152 |url=http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=830622}}</ref>

The melting point of gallium allows it to melt in the human hand, and then solidify if removed. The liquid metal has a strong tendency to [[supercooling|supercool]] below its [[melting point]]/[[freezing point]]: Ga [[nanoparticle]]s can be kept in the liquid state below 90&nbsp;K.<ref>{{cite journal |doi=10.1063/1.2221395 |title=Extreme undercooling (down to 90K) of liquid metal nanoparticles |journal=Applied Physics Letters |volume=89 |issue=3 |pages=033123 |year=2006 |last1=Parravicini |first1=G. B. |last2=Stella |first2=A. |last3=Ghigna |first3=P. |last4=Spinolo |first4=G. |last5=Migliori |first5=A. |last6=d'Acapito |first6=F. |last7=Kofman |first7=R. |bibcode=2006ApPhL..89c3123P}}</ref> [[Seed crystal|Seeding]] with a crystal helps to initiate freezing. Gallium is one of the four non-radioactive metals (with [[caesium]], [[rubidium]], and [[mercury (element)|mercury]]) that are known<!--PLEASE DO NOT ADD FRANCIUM; ITS MELTING POINT IS ONLY CALCULATED, AND ITS INTENSE RADIOACTIVITY WOULD MEAN THAT SHOULD YOU HAVE ENOUGH AROUND TO FILL A THERMOMETER, MEASURING ITS TEMPERATURE SHOULD NOT BE YOUR GREATEST CONCERN--> to be liquid at, or near, normal room temperature. Of the four, gallium is the only one that is neither highly reactive (as are rubidium and caesium) nor highly toxic (as is mercury) and can, therefore, be used in metal-in-glass high-temperature thermometers. It is also notable for having one of the largest liquid ranges for a metal, and for having (unlike mercury) a low [[vapor pressure]] at high temperatures. Gallium's boiling point, 2676&nbsp;K, is nearly nine times higher than its melting point on the [[Kelvin|absolute scale]], the greatest ratio between melting point and boiling point of any element.<ref name="GreenwoodEarnshaw2nd"/>{{rp|224}} Unlike mercury, liquid gallium metal [[wetting|wets]] glass and skin, along with most other materials (with the exceptions of quartz, graphite, [[gallium(III) oxide]]<ref>{{cite conference |last1=Chen |first1=Ziyu |last2=Lee |first2=Jeong-Bong |title=2019 IEEE 32nd International Conference on Micro Electro Mechanical Systems (MEMS) |chapter=Gallium Oxide Coated Flat Surface as Non-Wetting Surface for Actuation of Liquid Metal Droplets |year=2019 |pages=1–4 |doi=10.1109/memsys.2019.8870886| isbn=978-1-7281-1610-5 }}</ref> and [[PTFE]]),<ref name="GreenwoodEarnshaw2nd"/>{{rp|221}} making it mechanically more difficult to handle even though it is substantially less toxic and requires far fewer precautions than mercury. Gallium painted onto glass is a brilliant mirror.<ref name="GreenwoodEarnshaw2nd" />{{rp|221}} For this reason as well as the metal contamination and freezing-expansion problems, samples of gallium metal are usually supplied in polyethylene packets within other containers.

<div style="float:left;margin-right:0.5em;">
{|class="wikitable"
|+Properties of gallium for different crystal axes<ref name="anis" />
!Property!!''a''!! ''b'' !! ''c''
|-
|[[Thermal expansion|α]] (~25&nbsp;°C, μm/m)||16||11||31
|-
|[[Electrical resistivity and conductivity|ρ]] (29.7&nbsp;°C, nΩ·m)||543||174||81
|-
|ρ (0&nbsp;°C, nΩ·m)||480||154||71.6
|-
|ρ (77 K, nΩ·m)||101||30.8||14.3
|-
|ρ (4.2 K, pΩ·m)<!-- pico-Ohm, not a typo-->||13.8||6.8||1.6
|}</div>
Gallium does not [[crystal]]lize in any of the simple [[crystal structure]]s. The stable phase under normal conditions is [[orthorhombic]] with 8 atoms in the conventional [[unit cell]]. Within a unit cell, each atom has only one nearest neighbor (at a distance of 244&nbsp;[[picometre|pm]]). The remaining six unit cell neighbors are spaced 27, 30 and 39&nbsp;pm farther away, and they are grouped in pairs with the same distance.<ref>{{cite journal |last1=Bernasconi |first1=M. |last2=Chiarotti |first2=Guido L. |last3=Tosatti |first3=E. |title=Ab initio calculations of structural and electronic properties of gallium solid-state phases |journal=Physical Review B |date=October 1995 |volume=52 |issue=14 |pages=9988–9998 |doi=10.1103/PhysRevB.52.9988 |pmid=9980044 |bibcode=1995PhRvB..52.9988B }}</ref> Many stable and [[metastability in molecules|metastable]] phases are found as function of temperature and pressure.<ref>{{cite report |last1=Young |first1=David A. |title=Phase diagrams of the elements |date=11 September 1975 |id={{osti|4010212}} |doi=10.2172/4010212 }}</ref>

The bonding between the two nearest neighbors is [[covalent]]; hence Ga<sub>2</sub> [[Dimer (chemistry)|dimers]] are seen as the fundamental building blocks of the crystal. This explains the low melting point relative to the neighbor elements, aluminium and indium. This structure is strikingly similar to that of [[iodine]] and may form because of interactions between the single 4p electrons of gallium atoms, further away from the nucleus than the 4s electrons and the [Ar]3d<sup>10</sup> core. This phenomenon recurs with [[mercury (element)|mercury]] with its "pseudo-noble-gas" [Xe]4f<sup>14</sup>5d<sup>10</sup>6s<sup>2</sup> electron configuration, which is liquid at room temperature.<ref name="GreenwoodEarnshaw2nd"/>{{rp|223}} The 3d<sup>10</sup> electrons do not shield the outer electrons very well from the nucleus and hence the first ionisation energy of gallium is greater than that of aluminium.<ref name="GreenwoodEarnshaw2nd" />{{rp|222}} Ga<sub>2</sub> dimers do not persist in the liquid state and liquid gallium exhibits a complex low-coordinated structure in which each gallium atom is surrounded by 10 others, rather than 11–12 neighbors typical of most liquid metals.<ref>{{cite journal |last1=Yagafarov |first1=O. F. |last2=Katayama |first2=Y. |last3=Brazhkin |first3=V. V. |last4=Lyapin |first4=A. G. |last5=Saitoh |first5=H. |title=Energy dispersive x-ray diffraction and reverse Monte Carlo structural study of liquid gallium under pressure |journal=Physical Review B |date=7 November 2012 |volume=86 |issue=17 |page=174103 |doi=10.1103/PhysRevB.86.174103 |bibcode=2012PhRvB..86q4103Y }}</ref><ref>{{Cite journal |title=Structural Ordering in Liquid Gallium under Extreme Conditions |first1=James W. E.|last1=Drewitt |first2=Francesco |last2=Turci |first3=Benedict J. |last3=Heinen |first4=Simon G. |last4=Macleod |first5=Fei |last5=Qin |first6=Annette K. |last6=Kleppe |first7=Oliver T. |last7=Lord |date=9 April 2020 |journal=Physical Review Letters |volume=124 |issue=14 |pages=145501 |doi=10.1103/PhysRevLett.124.145501 |pmid=32338984 |bibcode=2020PhRvL.124n5501D |s2cid=216177238 |doi-access=free |hdl=1983/d385c37f-dc53-4177-985e-38875b57d8d9 |hdl-access=free}}</ref>

The physical properties of gallium are highly [[Anisotropy|anisotropic]], i.e. have different values along the three major crystallographic axes ''a'', ''b'', and ''c'' (see table), producing a significant difference between the linear (α) and volume [[thermal expansion]] coefficients. The properties of gallium are strongly temperature-dependent, particularly near the melting point. For example, the coefficient of thermal expansion increases by several hundred percent upon melting.<ref name="anis">{{cite book |author=Rosebury, Fred |title=Handbook of Electron Tube and Vacuum Techniques |url=https://books.google.com/books?id=yBmnnaODnHgC&pg=PA26 |date=1992 |publisher=Springer |isbn=978-1-56396-121-2 |page=26}}</ref>

===Isotopes===
{{Main|Isotopes of gallium}}
Gallium has 30 known isotopes, ranging in [[mass number]] from 60 to 89. Only two isotopes are stable and occur naturally, gallium-69 and gallium-71. Gallium-69 is more abundant: it makes up about 60.1% of natural gallium, while gallium-71 makes up the remaining 39.9%. All the other isotopes are radioactive, with gallium-67 being the longest-lived (half-life 3.261&nbsp;days). Isotopes lighter than gallium-69 usually decay through [[beta plus decay]] (positron emission) or [[electron capture]] to isotopes of [[zinc]], while isotopes heavier than gallium-71 decay through [[beta minus decay]] (electron emission), possibly with delayed [[neutron emission]], to isotopes of [[germanium]]. Gallium-70 can decay through both beta minus decay and electron capture. Gallium-67 is unique among the light isotopes in having only electron capture as a decay mode, as its decay energy is not sufficient to allow positron emission.<ref name="Audi">{{NUBASE 2003}}</ref> Gallium-67 and [[gallium-68]] (half-life 67.7&nbsp;min) are both used in [[nuclear medicine]].