# How are lowercase m and uppercase M used in general chemistry courses? [closed]

Where do lowercase m and uppercase M make appearances in general chemistry, and what do they signify?

Lowercase m

• $$m$$ is the official symbol for the quantity mass.
• m is an SI-prefix that stands for milli ($$\pu{e-3}$$), for example in mg (milligram).
• $$m$$ is sometimes uses as the symbol for molality. This is not IUPAC recommended (they recommend $$b$$) and leads to the confusing definition $$m_\text{solute} = n_\text{solute} / m_\text{solvent}$$ where the first $$m$$ is for molality and the second one for mass.

Uppercase M

• $$M$$ is used as symbol for the quantity molar mass (dimensions mass per amount of substance).

• M is an SI-prefix that stands for mega ($$\pu{e6}$$), for example in MW (megawatts).

• M is often used as an abbreviation for mol/L, and mM for mmol/L.

• $$M$$ is sometimes used as a symbol for the quantity molarity (dimensions are amount of substance per volume, official IUPAC name is amount of substance concentration, and official IUPAC symbol is $$c$$). Textbooks that follow this convention sometimes use MM or ℳ as an alternate symbol for molar mass to avoid name conflicts. Combining the quantity $$M$$ with the unit M gives silly statements such as: $$M = 5~\text{M}$$

Other useful information

• Make sure your lowercase m and uppercase M look different when you write or type, otherwise you are confusing your readers. If you type, quantities should be italicized and units should not. That also helps to distinguish the different meanings of m.
• You can't cancel m with m if they don't have the same meaning. It is fine to cancel the milli in mg/mL to obtain g/L, but not cancel $$m$$/mg to obtain 1/g.
• $\mathrm{M}$ is an old (my 1995 textbook already calls it that) abbreviation for mol/L for labelling bottles, like "$\mathrm{2M}$ HCl". It shouldn't be used in equations. The use of $M$ for molarity is really old. Profs still using that should be approached with a garlic bulb in one hand and a colt with a silver bullet in the other.
– Karl
May 13 '19 at 19:34
• @Karl This is not limited to old textbooks: chat.stackexchange.com/rooms/93601/… May 13 '19 at 20:32
• You get what you pay for, and the one you link is not the proverbial exception proving the rule ... Brr. My school textbooks (edition 1992) were better and more modern! They really use amu in an online chemistry textbook? When did they write that, in the late seventies?!? I regularily feel sorry for students from India (and other developing countries) asking questions here, for the outdated *#\$& they are taught, but the US?!?
– Karl
May 14 '19 at 20:27
• (sorry for the rant ;-))
– Karl
May 14 '19 at 20:27

### Lowercase

• m is a symbol for for meta- prefix (locant).
• m is a symbol for metastable [isotope].
• m is an abbreviation for "medium" or "multiplet" in spectroscopy.
• $$\pu{m}$$ sometimes used in place of $$\pu{mi}$$ for mile, e.g. mph — miles-per-hour.
• $$m$$ is a magnetic quantum number in NMR spectroscopy.

### Uppercase

• $$\ce{M}$$ signifies [central] metal (not $$\ce{Me}$$ — this one is methyl).
• M is used to denote mesomeric effect.
• M is a symbol for aminoacid methionine.
• M (minus, or Λ) is a left-handed helix in axial chirality.

### Small caps

• $$\mathrm{\small M}$$ is used for concentration (amount-of-substance concentration) of substance. It's rarely mentioned that when "M" is used in place of mol/L, it's supposed to be small caps "M" ($$\mathrm{\small M}$$), not just capital "M", but nobody seems to care. Also note that both the term and the symbol are deprecated [1, p. 27].

The term molarity and the symbol $$\mathrm{\small M}$$ should no longer be used because they, too, are obsolete. One should use instead amount-of-substance concentration of $$\ce{B}$$ and such units as $$\rm mol/dm^3$$, $$\rm kmol/m^3$$, or $$\rm mol/L$$.

### References

1. Thompson, A.; Taylor, B. N. Guide for the use of the international system of units (SI). NIST Special Publication 811 2008. (NIST)