Iron group

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.

Contents

Periodic table

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

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

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.

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
2He (an "alpha process") to the heavier elements present in the star, starting from 28
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 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
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 a Type II supernova.[9]

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

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

Nuclide stability

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

See also

Notes and references

Notes

  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

  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, http://books.google.co.uk/books?id=2rc5AAAAcAAJ .
  3. ^ "Proceedings of Societies [Report on the Law of Octaves]", Chemical News 13: 113, 1866, http://web.lemoyne.edu/~giunta/EA/NEWLANDSann.HTML#newlands5 .
  4. ^ Mendelejeff, D. (1869), "On the Relationship of the Properties of the Elements to their Atomic Weights", Z. Chem. 12: 405–6, http://web.lemoyne.edu/~giunta/EA/MENDELEEVann.HTML .
  5. ^ Mendeléeff, D. (1889), "The Periodic Law of the Chemical Elements", J. Chem. Soc. 55: 634–56, http://web.lemoyne.edu/~giunta/EA/MENDELann.HTML .
  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, A. (1984). Chemistry of the Elements. Oxford: Pergamon. 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 (I). Evaluation of input data, adjustment procedures", Nucl. Phys. A 729: 129–336, Bibcode 2003NuPhA.729..129W, doi:10.1016/j.nuclphysa.2003.11.002  Wapstra, A.H.; Audi, G.; Thibault, C. (2003), "The AME2003 atomic mass evaluation (II). Tables, graphs, and references", Nucl. Phys. A 729: 337–676, Bibcode 2003NuPhA.729..337A, doi:10.1016/j.nuclphysa.2003.11.003  Data tables.
  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: 633–730. Bibcode 2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633. http://physics.nist.gov/cuu/Constants/codata.pdf.  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 .