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 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.
In chemistry, "iron group" used to refer to iron and the next two elements in the periodic table namely cobalt and nickel, also called the "iron triad". Those 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. 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 similarities in chemistry were noted 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.
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+). 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.
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.
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 under a brief phase of "silicon burning". This involves the sequential addition of helium nuclei 4
(an "alpha process") to the heavier elements present in the star, starting from 28
All of these nuclear reactions are exothermic, that is they release energy: the energy that is released partially offsets the gravitational contraction of the star. However, the series ends at 56
, as the next reaction in the series,
|Nuclide mass||Mass defect||Binding energy|
|61.9283451(6) u||0.5700031(6) u||8.563872(10) MeV|
|57.9332756(8) u||0.5331899(8) u||8.563158(12) MeV|
|55.9349375(7) 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: 62
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 so-called S-process, the capture of slow neutrons within the star.
Notes and referencesEdit
- 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 = 60.0(11) years).
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