# 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.

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.

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

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 ndash 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

# superscript minus: U+207B: Cl⁻

EDIT

The situation, and how to realize the correct typography using $\LaTeX$ 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).

• Do you have an authoritative source for that I can show my group members when this comes up? – Canageek Apr 20 '15 at 4:12
• @Canageek Is the journal of DANTE, the German TeX User Association, good enough? I've updated my answer. – Klaus-Dieter Warzecha Apr 20 '15 at 6:24
• 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. – Canageek Apr 20 '15 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. – Klaus-Dieter Warzecha Apr 20 '15 at 19:55
• @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. – Klaus-Dieter Warzecha Apr 21 '15 at 9:38