iron group#Astrophysics

{{Periodic table (micro)|mark=Fe,Co,Ni|title=The iron group in the periodic table|caption=Fe, Co and Ni 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 periodic table, namely cobalt and nickel. These three comprised the "iron triad". They are the top elements of groups 8, 9, and 10 of the periodic table; or the top row of "group VIII" in the old (pre-1990) IUPAC system, or of "group VIIIB" in the CAS system.{{citation | last = Sherwood Taylor | first = F. | title = Inorganic and Theoretical Chemistry | edition = 6th | year = 1942 | location = London | publisher = Heinemann | pages = 151–54, 727–28}}. These three metals (and the three of the 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 ferromagnetism.

The similarities in chemistry were noted as one of Döbereiner's triads and by Adolph Strecker in 1859.{{citation | first = A. | last = Strecker | authorlink = Adolph 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}}. Indeed, Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel.{{citation | title = Proceedings of Societies [Report on the Law of Octaves] | journal = Chemical News | volume = 13 | page = 113 | year = 1866 | url = http://web.lemoyne.edu/~giunta/EA/NEWLANDSann.HTML#newlands5}}. Mendeleev stressed that groups of "chemically analogous elements" could have similar atomic weights as well as atomic weights which increase by equal increments, both in his original 1869 paper{{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}}. and his 1889 Faraday Lecture.{{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 }}.

In the traditional methods of qualitative inorganic analysis, the iron group consists of those cations which

• have soluble chlorides; and
• are not precipitated as sulfides by hydrogen sulfide in acidic conditions;
• are precipitated as hydroxides at around pH 10 (or less) in the presence of ammonia.

The main cations in the iron group are iron itself (Fe²⁺ and Fe³⁺), aluminium (Al³⁺) and chromium (Cr³⁺).{{VogelQualitative4th | pages = 260–78}}. If manganese is present in the sample, a small amount of hydrated manganese dioxide is often precipitated with the iron group hydroxides. Less common cations which are precipitated with the iron group include beryllium, titanium, zirconium, vanadium, uranium, thorium and cerium.{{VogelQualitative4th | pages = 592–611}}.

The iron group in astrophysics is the group of elements from chromium to nickel, which are substantially more abundant in the universe than those that come after them – or immediately before them – in order of atomic number.{{Greenwood&Earnshaw1st|pages=13–16}}. The study of the abundances of iron group elements relative to other elements in stars and supernovae allows the refinement of models of stellar evolution.

File:SolarSystemAbundances.svg. 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 nucleosynthesis in certain stars, specifically those of about 8–11 Solar masses. At the end of their lives, once other fuels have been exhausted, such stars can enter a brief phase of "silicon burning".{{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 }}. This involves the sequential addition of helium nuclei {{nuclide|link=yes|helium|4}} (an "alpha process") to the heavier elements present in the star, starting from {{nuclide|link=yes|silicon|28}}:

:

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

:

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 Type II supernova.

Nickel-56 is unstable with respect to beta decay, and the final stable product of silicon burning is {{nuclide|link=yes|iron|56}}.

:

It is often incorrectly stated that iron-56 is exceptionally common because it is the most stable of all the nuclides. This is not quite true: {{nuclide|link=yes|nickel|62}} and {{nuclide|link=yes|iron|58}} have slightly higher binding energies per nucleon – that is, they are slightly more stable as nuclides – as can be seen from the table on the right.{{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}}. 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 s-process, the capture of slow neutrons within the star.

File:Binding energy curve - common isotopes.svg 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.]]

• S-process
• Silicon burning process
• Abundance of the chemical elements

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Category:Sets of chemical elements

Category:Nucleosynthesis