Chemistry - Proper dash to use for negative charge

Solution 1:

[…] 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⁻

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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).

Solution 2:

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.