I have an alloy made of 50% iron and 50% cobalt. I see occasionally in the literature this is written as FeCo alloy.

But shouldn't it be CoFe alloy since the chemical symbols are supposed to be arranged in alphabetical order?

Is there somewhere where these rules are written down?

  • $\begingroup$ There is a lot of history and accustomed usage wrapped up in alloy names. Just the way it is (and no, having looked at many many phase diagrams I don't think I can come up with many rules. Well, hydrides always have the metal first, but then there is water, so it just goes to show... $\endgroup$ – Jon Custer Oct 1 '20 at 18:43
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    $\begingroup$ I will add that, for your particular alloy, it is shown as FeCo on the phase diagrams at the ASM Alloy Phase Diagram Database. But I don't think there is a signed treaty of understanding between metallurgists and chemists. $\endgroup$ – Jon Custer Oct 1 '20 at 19:08
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    $\begingroup$ If you want to become known as a IUPAC evangelist ... you will find a few guys on each side who are prepared to go for each other's throats on such issue, and the rest will be glad when they've strangled each other. ;-) $\endgroup$ – Karl Oct 1 '20 at 19:30
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    $\begingroup$ @Karl - yes, we need a new SALT treaty (Similar ALloy Terminology) to bring peace to the world... $\endgroup$ – Jon Custer Oct 1 '20 at 19:34
  • $\begingroup$ @JonCuster en.wikipedia.org/wiki/Contradictio_in_terminis :-)) $\endgroup$ – Karl Oct 1 '20 at 19:40

Current IUPAC recommendations suggest following electronegativity ordering. The symbol for iron (a less electronegative element among the two) should be written first: $\ce{FeCo}.$

From the section IR-4.4 Sequence of citation of symbols in formulae [1, pp. 58–59]:

IR-4.4.2 Ordering principles

IR- Electronegativity

If electronegativity is taken as the ordering principle in a formula or a part of a formula, the atomic symbols are cited according to relative electronegativities, the least electronegative element being cited first. For this purpose, Table VI* is used as a guide. By convention, the later an element occurs when the table is traversed following the arrows, the more electropositive is the element.


IR-4.4.3. Formulae for specific classes of compounds

IR- Binary species

In accordance with established practice, the electronegativity criterion (Section IR- is most often used in binary species.2


  1. For intermetallic compounds, earlier recommendations prescribed alphabetical ordering rather than by electronegativity (see Section I-4.6.6 of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 1990, ed. G.J. Leigh, Blackwell Scientific Publications, Oxford, 1990).



  1. $\ce{Rb15Hg16}$
  2. $\ce{Cu5Zn8}$ and $\ce{Cu5Cd8}$

Table VI as seen in [1, p. 260]:

Element sequence


  1. IUPAC. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; RSC Publishing: Cambridge, UK, 2005. ISBN 978-0-85404-438-2.

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