View source for Physics:Iron group

{{Short description|General term in chemistry and physics for the set of elements related to iron}}
{{hatnote|For the group headed by iron and additionally containing [[Chemistry:Ruthenium|ruthenium]], [[Chemistry:Osmium|osmium]], and [[Chemistry:Hassium|hassium]], see [[Chemistry:Group 8 element|group 8 element]].}}
In [[Chemistry:Chemistry|chemistry]] and [[Physics:Physics|physics]], the '''iron group''' refers to [[Chemistry:Chemical element|elements]] that are in some way related to [[Chemistry:Iron|iron]]; mostly in [[Chemistry:Period 4 element|period (row) 4]] of the periodic table. The term has different meanings in different contexts.

In chemistry, the term is largely obsolete, but it often means [[Chemistry:Iron|iron]], [[Chemistry:Cobalt|cobalt]], and [[Chemistry:Nickel|nickel]], also called the '''iron triad''';<ref name=green>M. Green, ed. (2002): ''[https://books.google.com/books?id=-QJWVxGinV4C&pg=PA285 Organometallic Chemistry]'', volume 10, page 283. Royal Society of Chemistry; 430 pages, {{isbn|9780854043330}}</ref> or, sometimes, other elements that resemble iron in some chemical aspects.  

In [[Physics:Astrophysics|astrophysics]] and [[Physics:Nuclear physics|nuclear physics]], the term is still quite common, and it typically means those three plus [[Chemistry:Chromium|chromium]] and [[Chemistry:Manganese|manganese]]—five elements that are exceptionally abundant, both on Earth and elsewhere in the universe, compared to their neighbors in the periodic table. [[Chemistry:Titanium|Titanium]] and [[Chemistry:Vanadium|vanadium]] are also produced in [[Astronomy:Type Ia supernova|Type Ia supernova]]e.<ref>{{cite journal|journal=Astronomy & Astrophysics|volume=550|title=Insights into thermonuclear supernovae from the incomplete Si-burning process|bibcode=2013A&A...550A..24B|url=https://www.aanda.org/articles/aa/abs/2013/02/aa20309-12/aa20309-12.html|last1=Bravo|first1=E.|year=2013|pages=A24|doi=10.1051/0004-6361/201220309|arxiv=1212.2410|s2cid=49331289 }}</ref>

==General chemistry==
{{Periodic table (micro)|mark=Fe,Ni,Co|title=The iron group in the [[Chemistry:Periodic table|periodic table]]|caption=Fe, Ni and Co are in group 8, 9, 10 (old group name VIII)}}
In chemistry, "iron group" used to refer to iron and the next two elements in the [[Chemistry:Periodic table|periodic table]], namely [[Chemistry:Cobalt|cobalt]] and [[Chemistry:Nickel|nickel]]. These three comprised the "iron triad".<ref name=green/> They are the top elements of [[Chemistry:Group (periodic table)|groups]] 8, 9, and 10 of the [[Chemistry:Periodic table|periodic table]]; or the top row of "group VIII" in the old (pre-1990) IUPAC system, or of "group VIIIB" in the [[Organization:Chemical Abstracts Service|CAS]] system.<ref>{{citation | last = Sherwood Taylor | first = F. | title = Inorganic and Theoretical Chemistry | edition = 6th | year = 1942 | location = London | publisher = Heinemann | pages = 151–54, 727–28}}.</ref> These three metals (and the three of the [[Earth:Platinum group|platinum group]], immediately below them) were set aside from the other elements because they have obvious similarities in their chemistry, but are not obviously related to any of the other groups. The iron group and its alloys exhibit [[Physics:Ferromagnetism|ferromagnetism]]. 

The similarities in chemistry were noted as one of [[Döbereiner's triads]] and by Adolph Strecker in 1859.<ref>{{citation | first = A. | last = Strecker |  title = Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente | location = Braunschweig | publisher = Friedrich Vieweg | year = 1859 | url = https://books.google.com/books?id=2rc5AAAAcAAJ}}.</ref> Indeed, Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel.<ref>{{citation | title = Proceedings of Societies [Report on the Law of Octaves&#93; | journal = Chemical News | volume = 13 | page = 113 | year = 1866 | url = http://web.lemoyne.edu/~giunta/EA/NEWLANDSann.HTML#newlands5}}.</ref> [[Biography:Dmitri Mendeleev|Mendeleev]] stressed that groups of "chemically analogous elements" could have similar [[Physics:Atomic weight|atomic weight]]s as well as atomic weights which increase by equal increments, both in his original 1869 paper<ref>{{citation | first = D. | last = Mendelejeff | authorlink = Dmitri Mendeleev | title = On the Relationship of the Properties of the Elements to their Atomic Weights | journal = Z. Chem. | volume = 12 | pages = 405–6 | year =1869 | url = http://web.lemoyne.edu/~giunta/EA/MENDELEEVann.HTML}}.</ref> and his 1889 Faraday Lecture.<ref>{{citation | title = The Periodic Law of the Chemical Elements | first = D. | last = Mendeléeff | authorlink = Dmitri Mendeleev | journal = J. Chem. Soc. | volume = 55 | pages = 634–56 | year = 1889 | url = http://web.lemoyne.edu/~giunta/EA/MENDELann.HTML | doi = 10.1039/ct8895500634 }}.</ref>

==Analytical chemistry==
In the traditional methods of qualitative inorganic analysis, the iron group consists of those cations which
*have soluble [[Chemistry:Chloride|chloride]]s; and
*are not precipitated as [[Chemistry:Sulfide|sulfide]]s by [[Chemistry:Hydrogen sulfide|hydrogen sulfide]] in [[Chemistry:Acid|acid]]ic conditions;
*are precipitated as [[Chemistry:Hydroxide|hydroxide]]s at around pH&nbsp;10 (or less) in the presence of [[Chemistry:Ammonia|ammonia]].
The main cations in the iron group are iron itself (Fe<sup>2+</sup> and Fe<sup>3+</sup>), [[Chemistry:Aluminium|aluminium]] (Al<sup>3+</sup>) and [[Chemistry:Chromium|chromium]] (Cr<sup>3+</sup>).<ref name="Vogel">{{VogelQualitative4th | pages = 260–78}}.</ref> If [[Chemistry:Manganese|manganese]] is present in the sample, a small amount of hydrated [[Chemistry:Manganese dioxide|manganese dioxide]] is often precipitated with the iron group hydroxides.<ref name="Vogel"/> Less common cations which are precipitated with the iron group include [[Chemistry:Beryllium|beryllium]], [[Chemistry:Titanium|titanium]], [[Chemistry:Zirconium|zirconium]], [[Chemistry:Vanadium|vanadium]], [[Chemistry:Uranium|uranium]], [[Chemistry:Thorium|thorium]] and [[Chemistry:Cerium|cerium]].<ref>{{VogelQualitative4th | pages = 592–611}}.</ref>

==Astrophysics==
The iron group in astrophysics is the group of elements from [[Chemistry:Chromium|chromium]] to [[Chemistry:Nickel|nickel]], which are substantially more abundant in the universe than those that come after them – or immediately before them – in order of [[Physics:Atomic number|atomic number]].<ref name="G&E">{{Greenwood&Earnshaw1st|pages=13–16}}.</ref> The study of the abundances of iron group elements relative to other elements in [[Astronomy:Star|star]]s and [[Astronomy:Supernova|supernova]]e allows the refinement of models of [[Astronomy:Stellar evolution|stellar evolution]].

[[File:SolarSystemAbundances.png|thumb|center|800px|Abundances of the chemical elements in the Solar System. The scale of the vertical axis is logarithmic. Hydrogen and helium are most common, from the [[Astronomy:Big Bang|Big Bang]]. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. The "iron peak" may be seen in the elements near iron as a secondary effect, increasing relative abundances of elements with nuclei most strongly bound.]]

The explanation for this relative abundance can be found in the process of [[Physics:Nucleosynthesis|nucleosynthesis]] in certain stars, specifically those of about 8–11&nbsp;[[Astronomy:Solar mass|Solar mass]]es. At the end of their lives, once other fuels have been exhausted, such stars can enter a brief phase of "silicon burning".<ref name="Siburn">{{citation | first1 = Stan | last1 = Woosley | first2 = Thomas | last2 = Janka | title = The Physics of Core-Collapse Supernovae | arxiv = astro-ph/0601261 | journal = Nature Physics | volume = 1 | issue = 3 | year = 2005 | pages = 147–54 | doi = 10.1038/nphys172 | bibcode=2005NatPh...1..147W| citeseerx = 10.1.1.336.2176 | s2cid = 118974639 }}.</ref> This involves the sequential addition of [[Chemistry:Helium|helium]] nuclei {{nuclide|link=yes|helium|4}} (an "[[Astronomy:Alpha process|alpha process]]") to the heavier elements present in the star, starting from {{nuclide|link=yes|silicon|28}}:

<!-- Autogenerated using Phykiformulae 0.12 User:SkyLined#Phykiformulae
Si-28 + He-4 -> S-32
S-32  + He-4 -> Ar-36
Ar-36 + He-4 -> Ca-40
Ca-40 + He-4 -> Ti-44
Ti-44 + He-4 -> Cr-48
Cr-48 + He-4 -> Fe-52
Fe-52 + He-4 -> Ni-56
-->:{| border="0"
|- style="height:2em;"
|{{nuclide|silicon|28}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|sulfur|32}}
|- style="height:2em;"
|{{nuclide|sulfur|32}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|argon|36}}
|- style="height:2em;"
|{{nuclide|argon|36}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|calcium|40}}
|- style="height:2em;"
|{{nuclide|calcium|40}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|titanium|44}}&nbsp;<ref group="note">In lighter stars, with less gravitational pressure, the alpha process is much slower and effectively stops at this stage as titanium-44 is unstable with respect to beta decay (t<sub>1/2</sub>&nbsp;= 60.0(11)&nbsp;years).</ref>
|- style="height:2em;"
|{{nuclide|titanium|44}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|chromium|48}}
|- style="height:2em;"
|{{nuclide|chromium|48}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|iron|52}}
|- style="height:2em;"
|{{nuclide|iron|52}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|nickel|56}}
|}
All of these nuclear reactions are exothermic: the energy that is released partially offsets the gravitational contraction of the star. However, the series ends at {{nuclide|nickel|56}}, as the next reaction in the series
:{| border="0"
|- style="height:2em;"
|{{nuclide|nickel|56}}&nbsp;||+&nbsp;||{{nuclide|helium|4}}&nbsp;||→&nbsp;||{{nuclide|link=yes|zinc|60}}
|}
is endothermic. With no further source of energy to support itself, the core of the star collapses on itself while the outer regions are blown off in a [[Astronomy:Type II supernova|Type&nbsp;II]] [[Astronomy:Supernova|supernova]].<ref name="Siburn"/>

Nickel-56 is unstable with respect to [[Physics:Beta decay|beta decay]], and the final stable product of silicon burning is {{nuclide|link=yes|iron|56}}.
:{| border="0"
|- style="height:2em;"
|{{nuclide|nickel|56}}&nbsp;||→&nbsp;||{{nuclide|link=yes|cobalt|56}}&nbsp;||+&nbsp;||[[Physics:Positron|β<sup>+</sup>]]||&nbsp;&nbsp;t<sub>1/2</sub>&nbsp;=&nbsp;6.075(10)&nbsp;d
|- style="height:2em;"
|{{nuclide|cobalt|56}}&nbsp;||→&nbsp;||'''{{nuclide|iron|56}}'''&nbsp;||+&nbsp;||β<sup>+</sup>||&nbsp;&nbsp;t<sub>1/2</sub>&nbsp;=&nbsp;77.233(27)&nbsp;d
|}

{| class=wikitable align=right style="margin:0 0 0 0.5em;"
|-
! &nbsp; !! Nuclide mass<ref name="AME">{{AME 2003}}</ref> !! Mass defect<ref name="Amsler">{{citation | author = Particle Data Group | year = 2008 | title = Review of Particle Physics | journal = Phys. Lett. B | volume = 667 | issue = 1–5 | pages = 1–6 | doi = 10.1016/j.physletb.2008.07.018 | bibcode = 2008PhLB..667....1A |  url = https://www.zora.uzh.ch/id/eprint/11249/2/scalarsV.pdf | hdl = 1854/LU-685594 | s2cid = 227119789 | hdl-access = free | access-date = 2019-12-13 | archive-date = 2020-09-07 | archive-url = https://web.archive.org/web/20200907211544/https://www.zora.uzh.ch/id/eprint/11249/2/scalarsV.pdf | url-status = dead }}. [http://pdg.lbl.gov/ Data tables].</ref> !! Binding energy<br/>per nucleon<ref name="CODATA">{{CODATA2006|url=http://physics.nist.gov/cgi-bin/cuu/Convert?exp=0&num=1&From=u&To=ev&Action=Convert+value+and+show+factor}}</ref>
|-
| {{nuclide|nickel|62}} || 61.9283451(6)&nbsp;u || 0.5700031(6)&nbsp;u || 8.563872(10)&nbsp;MeV
|-
| {{nuclide|iron|58}} || 57.9332756(8)&nbsp;u || 0.5331899(8)&nbsp;u || 8.563158(12)&nbsp;MeV
|-
| {{nuclide|iron|56}} || 55.9349375(7)&nbsp;u || 0.5141981(7)&nbsp;u || 8.553080(12)&nbsp;MeV
|-
|}
It is often incorrectly stated that iron-56 is exceptionally common because it is the most stable of all the nuclides.<ref name="G&E"/> This is not quite true: {{nuclide|link=yes|nickel|62}} and {{nuclide|link=yes|iron|58}} have slightly higher [[Physics:Binding energy|binding energies per nucleon]] – that is, they are slightly more stable as nuclides – as can be seen from the table on the right.<ref>{{citation | title = The atomic nuclide with the highest mean binding energy | last = Fewell | first = M. P. | journal = Am. J. Phys. | volume = 63 | issue = 7 | year = 1995 | pages = 653–58 | doi = 10.1119/1.17828 | bibcode=1995AmJPh..63..653F}}.</ref> However, there are no rapid nucleosynthetic routes to these nuclides. 

In fact, there are several stable nuclides of elements from chromium to nickel around the top of the stability curve, accounting for their relative abundance in the universe. The nuclides which are not on the direct alpha-process pathway are formed by the [[Physics:S-process|s-process]], the capture of slow [[Physics:Neutron|neutron]]s within the star.
[[File:Binding energy curve - common isotopes.svg|thumb|center|500px|The curve of [[Physics:Binding energy|binding energy]] per nucleon (calculated from the nuclear mass defect) against the number of nucleons in the nucleus. Iron-56 is labelled near the very top of the curve: it can be seen that the "peak" is quite flat, which explains the existence of several common elements around iron.]]

==See also==
* [[Astronomy:Sun#Singly ionized iron-group elements|Singly ionized iron-group elements]]
* [[Physics:S-process|S-process]]
* Silicon burning process
* [[Chemistry:Abundance of the chemical elements|Abundance of the chemical elements]]

==Notes and references==

===Notes===
{{reflist|group=note}}

===References===
{{reflist|30em}}

{{Periodic table (navbox)}}
[[Category:Sets of chemical elements]]
[[Category:Nucleosynthesis]]

{{Sourceattribution|Iron group}}