Iron group

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Monument to Dmitri Mendeleev in Saint Petersburg, Russia. Group VIII is on the right-hand side of the old style periodic table.

In chemistry and physics, the iron group refers to elements that are in some way related to iron. These elements are relatively abundant both on Earth and elsewhere in the universe. The term is ambiguous in different contexts, and almost obsolete in chemistry.

Periodic table[edit]

The iron group in the periodic table referred to the elements iron, cobalt and nickel, that is the first row of group VIII (or VIIIB) under the old numbering system.[1] These metals, and the platinum group immediately below them, were set aside from the other elements as they show obvious similarities among themselves in their chemistry, but are not obviously related to any of the other groups.

The similarities in chemistry along what is now known as the first row of the transition metals were noted by Adolph Strecker in 1859.[2] Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel.[3] 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[4] and his 1889 Faraday Lecture.[5]

Analytical chemistry[edit]

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+) and chromium (Cr3+).[6] If manganese is present in the sample, a small amount of hydrated manganese dioxide is often precipitated with the iron group hydroxides.[6] Less common cations which are precipitated with the iron group include beryllium, titanium, zirconium, vanadium, uranium, thorium and cerium.[7]

Astrophysics[edit]

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.[8] 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.

Abundances of the chemical elements in the Solar System. Note that the scale of the vertical axis is logarthmic. Hydrogen and helium are most common, from the 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 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".[9] This involves the sequential addition of helium nucli 4
2
He
(an "alpha process") to the heavier elements present in the star, starting from 28
14
Si
:

28
14
Si
 
4
2
He
 
→  32
16
S
32
16
S
 
4
2
He
 
→  36
18
Ar
36
18
Ar
 
4
2
He
 
→  40
20
Ca
40
20
Ca
 
4
2
He
 
→  44
22
Ti
 [note 1]
44
22
Ti
 
4
2
He
 
→  48
24
Cr
48
24
Cr
 
4
2
He
 
→  52
26
Fe
52
26
Fe
 
4
2
He
 
→  56
28
Ni

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
28
Ni
, as the next reaction in the series,

56
28
Ni
 
4
2
He
 
→  60
30
Zn

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.[9]

Nickel-56 is unstable with respect to beta decay, and the final stable product of silicon burning is 56
26
Fe
.

56
28
Ni
 
→  56
27
Co
 
β+   t½ = 6.075(10) d
56
27
Co
 
→  56
26
Fe
 
β+   t½ = 77.233(27) d

Nuclide stability[edit]

  Nuclide mass[10] Mass defect[11] Binding energy
per nucleon[12]
62
28
Ni
61.9283451(6) u 0.5700031(6) u 8.563872(10) MeV
58
26
Fe
57.9332756(8) u 0.5331899(8) u 8.563158(12) MeV
56
26
Fe
55.9349375(7) u 0.5141981(7) u 8.553080(12) MeV

It is often incorrectly assumed that iron-56 is the most stable of all the nuclides.[8] This is not quite true: 62
28
Ni
and 58
26
Fe
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.[13] However, there are no rapid nucleosynthetic routes to these nuclides. 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.

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.

See also[edit]

Notes and references[edit]

Notes[edit]

  1. ^ 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 (t½ = 60.0(11) years).

References[edit]

  1. ^ Sherwood Taylor, F. (1942), Inorganic and Theoretical Chemistry (6th ed.), London: Heinemann, pp. 151–54, 727–28 .
  2. ^ Strecker, A. (1859), Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente, Braunschweig: Friedrich Vieweg .
  3. ^ "Proceedings of Societies [Report on the Law of Octaves]", Chemical News 13, 1866: 113 .
  4. ^ Mendelejeff, D. (1869), "On the Relationship of the Properties of the Elements to their Atomic Weights", Z. Chem. 12: 405–6 .
  5. ^ Mendeléeff, D. (1889), "The Periodic Law of the Chemical Elements", J. Chem. Soc. 55: 634–56 .
  6. ^ a b Vogel, Arthur I. (1954), A Textbook of Macro and Semimicro Qualitative Inorganic Analysis (4th ed.), London: Longman, pp. 260–78, ISBN 0-582-44367-9 .
  7. ^ Vogel, Arthur I. (1954), A Textbook of Macro and Semimicro Qualitative Inorganic Analysis (4th ed.), London: Longman, pp. 592–611, ISBN 0-582-44367-9 .
  8. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 13–16. ISBN 0-08-022057-6. .
  9. ^ a b Woosley, Stan; Janka, Thomas (2005), "The Physics of Core-Collapse Supernovae", Nature Physics 1 (3): 147–54, arXiv:astro-ph/0601261, Bibcode:2005NatPh...1..147W, doi:10.1038/nphys172 .
  10. ^ Wapstra, A.H.; Audi, G.; Thibault, C. (2003), The AME2003 Atomic Mass Evaluation (Online ed.), National Nuclear Data Center . Based on:
  11. ^ Particle Data Group (2008), "Review of Particle Physics", Phys. Lett. B 667 (1–5): 1–6, Bibcode:2008PhLB..667....1P, doi:10.1016/j.physletb.2008.07.018 . Data tables.
  12. ^ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006". Rev. Mod. Phys. 80 (2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633.  Direct link to value.
  13. ^ Fewell, M. P. (1995), "The atomic nuclide with the highest mean binding energy", Am. J. Phys. 63 (7): 653–58, Bibcode:1995AmJPh..63..653F, doi:10.1119/1.17828 .