Proper dash to use for negative charge

What type of dash does one use when indicating something like F$^{-}$? I assumed it was a minus, since you indicate positive charge with a +, but there is one professor I know who insists that it should be typeset with an endash. Is there a style manual on chemical typesetting I can consult.

• Say for instance in Microsoft Word, if you compare the minus sign from the table of symbols (which is the correct symbol) to a hyphen, an en dash, and an em dash. You may find it quite difficult to distinguish between the legitimate minus symbol and the en dash. The hyphen and the em dash easily look like sloppy choices for an alternative. Feb 5, 2021 at 22:32

2 Answers

[…] one professor I know who insists that it should be typeset with an en dash.

I disagree! A standard minus sign should be

• on the same height as the horizontal line of a plus sign;
• as long as an equal sign.

The en dash is

• lower than the horizontal bar of the plus sign;
• slightly longer than the equal sign.

Unicode has dedicated characters for minus and superscript minus

• standard minus: U+2212: −
• superscript minus: U+207B: Cl⁻

See also:
The situation, and how to realize the correct typography using $$\mathrm\LaTeX$$ has been described in the German article Feinheiten bei wissenschaftlichen Publikationen – Mikrotypographie-Regeln, Teil II by Marion Neubauer in Die TeXnische Kommödie, 1997, 1, 25-44 (PDF).

• I've been talking with some people on the TeX Stackexchange. It seems that in most fonts hyphen-minus, the keyboard symbol, is the length of a hyphen, not a minus. So the correct solution is to manually insert U+2212 (MINUS SIGN). However if that is not available, the endash is usually the same length as it and is a better replacement then hyphen-minus. Apr 20, 2015 at 19:49
• @Canageek In $\LaTeX$, things are easy :) Within equations in math mode, a - is automatically rendered in the right length and with the right vertical position. The same is true for a superscripted minus within a \ce{...} expression. Apr 20, 2015 at 19:55
• @Canageek To quote the Red Book (Nomenclature of Inorganic Chemistry) by the International Union of Applied Chemistry (IUPAC), rule IR 2.3.2 "Plus and minus signs" their statement is: "The signs $+$ and $-$ are used to indicate the charge on an ion in a formula or name. Compare with this old.iupac.org/publications/books/rbook/Red_Book_2005.pdf, page 25. Not a iota of "dash". Apr 21, 2015 at 8:26
• @Buttonwood There's not doubt about what a minus sign is used for :-) The "problem" in typography arises from the fact that there is no real minus sign on an ASCII keyboard. It actually is a hyphen. Apr 21, 2015 at 9:38
• "The ndash is - lower than the horizontal bar of the plus sign - slightly longer than the equal sign." Actually, neither of those is always the case. It depends on the font. Yes, the en dash is usually slightly longer than a true minus sign. But using Helvetica in Word-for-Mac, for instance, it's 1% shorter. And as for the height, there is no general rule. In Word-for-Mac, the en dash is lower with Arial and Times, nearly dead-even with Calibri, and higher with Helvetica. Feb 6, 2021 at 5:08

On this matter, the Green Book (‘Quantities, Units and Symbols in Physical Chemistry’) refers to the Red Book (‘Nomenclature of Inorganic Chemistry – IUPAC recommendations 2005’), which clearly states that plus and minus signs are used to indicate the charge on an ion in a formula or name.

However, this statement does not clarify which particular character shall be used as minus sign. Nevertheless, the Red Book also explicitly mentions hyphens and ‘em’ dashes for other purposes but no ‘en’ dashes. Hence, I suppose, the Red Book would have mentioned ‘en’ dashes if this character shall be used.

Fortunately, signs and symbols for quantities and units etc are standardized in the international standard series ISO/IEC 80000, which has been adopted by many national standards. This standard is intended mainly for use in the natural sciences and technology. ISO 80000 Part 2 (‘Mathematical signs and symbols to be used in the natural sciences and technology’) clarifies precisely that the symbol that shall be used as minus sign really is the MINUS SIGN of ISO/IEC 10646 and Unicode (they are identical). The hexadecimal code assigned to the minus sign by both ISO/IEC 10646 and Unicode is 2212.