Inchemistry andphysics, theiron group refers toelements that are in some way related toiron; mostly inperiod (row) 4 of the periodic table. The term has different meanings in different contexts.
In chemistry, the term is largely obsolete, but it often meansiron,cobalt, andnickel, also called theiron triad;[1]. It may sometimes refer to other elements that resemble iron in some chemical aspects, such as the stablegroup 8 elements (Fe,Ru,Os).[2][3]
Inastrophysics andnuclear physics, the term is still quite common, and it typically means those three pluschromium andmanganese—five elements that are exceptionally abundant, both on Earth and elsewhere in the universe, compared to their neighbors in the periodic table.Titanium andvanadium are also produced inType Ia supernovae.[4]
In chemistry, "iron group" used to refer to iron and the next two elements in theperiodic table, namelycobalt andnickel. These three comprised the "iron triad".[1] They are the top elements ofgroups 8, 9, and 10 of theperiodic table; or the top row of "group VIII" in the old (pre-1990) IUPAC system, or of "group VIIIB" in theCAS system.[5] These three metals (and the three of theplatinum 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 itsalloys exhibitferromagnetism.
The similarities in chemistry were noted as one ofDöbereiner's triads and byAdolph Strecker in 1859.[6] Indeed,Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel.[7]Mendeleev stressed that groups of "chemically analogous elements" could have similaratomic weights as well as atomic weights which increase by equal increments, both in his original 1869 paper[8] and his 1889Faraday Lecture.[9]
In the traditional methods of qualitative inorganic analysis, the iron group consists of those cations which
The main cations in the iron group are iron itself (Fe2+ and Fe3+),aluminium (Al3+) andchromium (Cr3+).[10] Ifmanganese is present in the sample, a small amount of hydratedmanganese dioxide is often precipitated with the iron group hydroxides.[10] Less common cations which are precipitated with the iron group includeberyllium,titanium,zirconium,vanadium,uranium,thorium andcerium.[11]
The iron group in astrophysics is the group of elements fromchromium tonickel, which are substantially more abundant in the universe than those that come after them – or immediately before them – in order ofatomic number.[12] The study of the abundances of iron group elements relative to other elements instars andsupernovae allows the refinement of models ofstellar evolution.
The explanation for this relative abundance can be found in the process ofnucleosynthesis 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".[13] This involves the sequential addition ofhelium nuclei4
2He
(an "alpha process") to the heavier elements present in the star, starting from28
14Si
:
| 28 14Si | + | 4 2He | → | 32 16S |
| 32 16S | + | 4 2He | → | 36 18Ar |
| 36 18Ar | + | 4 2He | → | 40 20Ca |
| 40 20Ca | + | 4 2He | → | 44 22Ti [note 1] |
| 44 22Ti | + | 4 2He | → | 48 24Cr |
| 48 24Cr | + | 4 2He | → | 52 26Fe |
| 52 26Fe | + | 4 2He | → | 56 28Ni |
All of these nuclear reactions areexothermic: the energy that is released partially offsets the gravitational contraction of the star. However, the series ends at56
28Ni
, as the next reaction in the series
| 56 28Ni | + | 4 2He | → | 60 30Zn |
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 aType IIsupernova.[13]
Nickel-56 is unstable with respect tobeta decay, and the final stable product of silicon burning is56
26Fe
.
| Nuclide mass[14] | Mass defect[15] | Binding energy per nucleon[16] | |
|---|---|---|---|
| 62 28Ni | 61.9283448(5) u | 0.5700031(6) u | 8.563872(10) MeV |
| 58 26Fe | 57.9332736(3) u | 0.5331899(8) u | 8.563158(12) MeV |
| 56 26Fe | 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.[12] This is not quite true:62
28Ni
and58
26Fe
have slightly higherbinding energies per nucleon – that is, they are slightly more stable as nuclides – as can be seen from the table on the right.[17] 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 thes-process, the capture of slowneutrons within the star.
