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.

Periodic table

The iron group in the periodic table

Fe, Ni and Co are in group VIII (8, 9, 10)

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]) In modern numbering, the iron group appears as three columns numbered group 8, 9 and 10. 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

• 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⁺).^[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.

Abundances of the chemical elements in the Solar System. Note that the scale of the vertical axis is logarthimic. 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 nuclei 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+	t1/2 = 6.075(10) d
56 27Co	→	56 26Fe	+	β+	t1/2 = 77.233(27) d

Nuclide stability

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.

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

• Singly ionized iron group elements
• S-process
• Silicon burning process
• Abundance of the chemical elements

Notes and references

Notes

References