Iron group

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General term in chemistry and physics for the set of elements related to iron

title: "Iron group" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["sets-of-chemical-elements", "nucleosynthesis"] description: "General term in chemistry and physics for the set of elements related to iron" topic_path: "science/chemistry" source: "https://en.wikipedia.org/wiki/Iron_group" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary General term in chemistry and physics for the set of elements related to iron ::

In chemistry and physics, the iron group refers to elements that are in some way related to iron; mostly in 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 iron, cobalt, and nickel, also called the iron triad;. It may sometimes refer to other elements that resemble iron in some chemical aspects, such as the stable group 8 elements ().

In astrophysics and nuclear physics, the term is still quite common, and it typically means those three plus chromium and manganese—five elements that are exceptionally abundant, both on Earth and elsewhere in the universe, compared to their neighbors in the periodic table. Titanium and vanadium are also produced in Type Ia supernovae.

General chemistry

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". 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. Indeed, Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel. 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 and his 1889 Faraday Lecture.

Analytical chemistry

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

Astrophysics

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. 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.

::figure[src="https://upload.wikimedia.org/wikipedia/commons/9/9a/SolarSystemAbundances.svg" caption="nuclei]] most strongly bound."] ::

The explanation for this relative abundance can be found in the process of nucleosynthesis in certain stars, specifically those with more than 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". This involves the sequential addition of helium nuclei (an "alpha process") to the heavier elements present in the star, starting from :

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;" | ||+ || ||→ || |- style="height:2em;" | ||+ || ||→ || |- style="height:2em;" | ||+ || ||→ || |- style="height:2em;" | ||+ || ||→ || 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 (t1/2 = 59.1 years). |- style="height:2em;" | ||+ || ||→ || |- style="height:2em;" | ||+ || ||→ || |- style="height:2em;" | ||+ || ||→ || |} 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 , as the next reaction in the series : {| border="0" |- style="height:2em;" | ||+ || ||→ || |} 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 . : {| border="0" |- style="height:2em;" | ||→ || ||+ ||β+|| t1/2 = 6.075 d |- style="height:2em;" | ||→ ||**** ||+ ||β+|| t1/2 = 77.24 d |}

::data[format=table] | | Nuclide mass | 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 | author-link = Particle Data Group | 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 }}. Data tables. | author-last1=Mohr |author-first1=Peter J. |author-first2=Barry N. |author-last2=Taylor |author-first3=David B. |author-last3=Newell |year=2008 |issue=2 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2006 |url=https://www.nist.gov/pml/div684/fcdc/upload/rmp2006-2.pdf |journal=Reviews of Modern Physics |volume=80 |doi=10.1103/RevModPhys.80.633 |pages=633–730 |bibcode=2008RvMP...80..633M |arxiv=0801.0028 |archive-url=https://web.archive.org/web/20171001121533/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp2006-2.pdf|archive-date=2017-10-01}} | |---|---|---|---| | | 61.9283448(5) u | 0.5700031(6) u | 8.563872(10) MeV | | | 57.9332736(3) u | 0.5331899(8) u | 8.563158(12) MeV | | | 55.93493554(29) u | 0.5141981(7) u | 8.553080(12) MeV | ::

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: and 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. 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. ::figure[src="https://upload.wikimedia.org/wikipedia/commons/5/53/Binding_energy_curve_-_common_isotopes.svg" caption="The curve of [[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."] ::

References

  1. 9780854043330
  2. (1986). "Conductive polymers derived from iron, ruthenium, and osmium metalloporphyrins: The shish-kebab approach". Proceedings of the National Academy of Sciences.
  3. (1977). "Synthesis of tetrahedral mixed-metal clusters of the iron triad. Preparation and characterization of tridecacarbonyldihydroironosmiumdiruthenium and tridecacarbonyldihydroirondiosmiumruthenium". Journal of the American Chemical Society.
  4. (2013). "Insights into thermonuclear supernovae from the incomplete Si-burning process". Astronomy & Astrophysics.
  5. Sherwood Taylor, F.. (1942). "Inorganic and Theoretical Chemistry". Heinemann.
  6. Strecker, A.. (1859). "Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente". Friedrich Vieweg.
  7. (1866). ["Proceedings of Societies Report on the Law of Octaves]". Chemical News.
  8. Mendelejeff, D.. (1869). "On the Relationship of the Properties of the Elements to their Atomic Weights". Z. Chem..
  9. Mendeléeff, D.. (1889). "The Periodic Law of the Chemical Elements". J. Chem. Soc..
  10. {{VogelQualitative4th
  11. {{VogelQualitative4th
  12. {{Greenwood&Earnshaw1st
  13. (2005). "The Physics of Core-Collapse Supernovae". Nature Physics.
  14. Particle Data Group. (2008). "Review of Particle Physics". Phys. Lett. B.
  15. Fewell, M. P.. (1995). "The atomic nuclide with the highest mean binding energy". Am. J. Phys..

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sets-of-chemical-elementsnucleosynthesis