Does anyone know if there is an officially sanctioned way to typeset symbols like technetium-99m (99Tcm or 99mTc)? I have seen both, although in more recent publications, I think, the latter predominates.

For my part, it looks somewhat wrong to place the symbol for metastable in the position you'd normally expect the charge. How would you write the ion, 99Tcm+? That looks quite clumsy.

So does anyone know if there's some IUPAC (or similarly authoritative) guidance on this?


According to the international standard ISO 80000 Quantities and units – Part 9: Physical chemistry and molecular physics, Amendment 1, a metastable nuclide is indicated by adding the letter m (in roman type) to the mass number of the nuclide, as in the following examples:

$$\mathrm{^{133m}Xe}$$ $$\mathrm{^{99m}Tc^+}$$

  • $\begingroup$ I'm quite confused since BS ISO 80000-9:2009 (Annex B, p. 34) suggests otherwise: "State of nuclear excitation: $\ce{^110Ag^*}$ or $\ce{^110Ag^\mathrm{m}}$". Are you citing a more recent ISO, maybe? $\endgroup$
    – andselisk
    Jan 17 '18 at 23:26
  • 1
    $\begingroup$ @andselisk Yes indeed. The first edition of ISO 80000-9 (2009-04-01) had in Annex B: “$\ce{^110Ag^*}$ or $\ce{^110Ag^\mathrm{m}}$”. This mistake was corrected in Amendment 1 (2011-06-01). $\endgroup$
    – user7951
    Feb 11 '18 at 13:34

According to IUPAP recommendations (Symbols, Units, Nomenclature and Fundamental Constants in Physics), part 2 SYMBOLS FOR ELEMENTS, PARTICLES, STATES AND TRANSITIONS, 2.1 Chemical elements, both placements are valid [1, p. 9]:

The right superscript position should be used, if required, to indicate a state of ionization (e.g., $\ce{Ca^2+}$, $\ce{PO4^3-}$) or an excited atomic state (e.g., $\ce{He^∗}$). A metastable nuclear state, however, often is treated as a distinct nuclide: e.g., either $\ce{^118Ag^\mathrm{m}}$ or $\ce{^\mathrm{118m}Ag}$.

On the other hand, AIP Style Guide in Part III. General style, A. GRAMMAR AND PUNCTUATION, 12. Symbols for nuclides suggests that roman "m" is placed as superscript on the right [2, p. 16]:

Journals of AIP and its Member Societies follow the recommendations of the Symbols, Units, and Nomencla­ture (S.U.N.) Commission of the International Union of Pure and Applied Physics on the symbols to be used for nuclides and their states. [...]

A posterior superscript can indicate either a state of ioni­zation: $$\ce{Ca^2+}$$ or an excited state: $$\ce{^110Ag^\mathrm{m}},~\ce{^14N^*}$$


  1. Cohen, R. E.; Giacomo, P. IUPAP “Red Book” Symbols, Units, Nomenclature and Fundamental Constants in Physics (2010 Reprint); IUPAP recommendations; Commission C2 – Sunamco, 1987.
  2. AIP Style Manual, 4th ed.; American Institute of Physics, AIP Publication Board, Eds.; American Institute of Physics: New York, N.Y., 1990. ISBN 978-0-88318-642-8.

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