# Which symbols are written in roman (upright) font and which are italicized?

This post tries to be a guide to new members of the (chemical) scientific community for instructions on how to apply the right formatting when writing chemical and mathematical equations.

• As a heavy editor myself, I am absolutely pro nice typeset. So while I see a post that needs redo, I try to go the whole ten yards, i.e. also changing all \times to \cdot. Whenever I review an edit that contains these faulty typesets I will improve it. When I see some faulty typeset in a post, but it is still very much understandable, I usually leave it that way (some people just refuse to use mhchem and then roll back). I think a good typeset is important for future references, so it should be done. – Martin - マーチン Jul 29 '14 at 16:32
• @Martin-マーチン In many cases, changing \times to \cdot may be acceptable, preferable, or even absolutely necessary; however “if the point is used as the decimal sign, the cross and not the half-high dot should be used as the multiplication sign between numbers expressed with digits.” $4\,711.32 \times 0.351\,2$ – Faded Giant Jul 18 '15 at 8:10

Which symbols are written in roman (upright) font?

As a rule of thumb: Symbols representing physical quantities or mathematical variables are the only things written in italic type.

## Guidelines

Everything of the following is not written in italics:

• Unit symbols, e.g. $$\mathrm{kg}$$, $$\mathrm{kJ}$$, $$\mathrm{mol}$$, $$\mathrm{K}$$
• Chemical formulae. These are written in roman font automatically when using the mhchem commands, i.e. $\ce{...}$.
• Function names such as $$\sin$$, $$\cos$$, $$\log$$, i.e. \sin, \cos, \log. (MathJax recognizes standard mathematical function names and automatically applies the correct style to them.)
• Mathematical constants, the values of which never change, e.g. $$\mathrm{e} = 2.718\,218\,8\ldots$$, $$\mathrm {i}^2 = -1$$
(Note that this convention for $$\mathrm e$$ and $$\mathrm i$$ is recommended by ISO 80000, IUPAC, NIST, and ACS, but italicizing these expressions is also very common.)
• Descriptive indices such as $$_\text{ox}$$, $$_\text{red}$$ or $$_\text{tot}$$
• Descriptive text
• Symbols for mathematical operators, e.g. $$\Delta$$ in $$\Delta x=x_2-x_1$$ and each $$\mathrm d$$ in $$\mathrm df/\mathrm dx$$ (derivative of $$f$$ with respect to $$x$$). Note that $$x$$ and $$y$$ are variables in this context.
(Note that this convention for differentials and derivatives is recommended by ISO 80000, IUPAC, NIST, and ACS; it is also used in the rules and style conventions presented in The International System of Units (SI). Nevertheless, italicizing these expressions is also very common.)
• Electronic configurations $$\mathrm{(1s)^2 (2s)^2 (2p)^4}$$

Everything of the following is written in italics:

• Symbols representing physical quantities, e.g. $$m$$ for mass or $$V$$ for volume,
including fundamental physical constants (quantities that are considered to be constant under all circumstances), e.g. Planck constant $$h$$, Faraday constant $$F$$
• Mathematical variables, e.g. $$x$$ and $$y$$, including expressions such as “the $$x$$ axis”
• Iterative variables such as $$i$$ in a sum
• Parameters, such as $$a$$, $$b$$, etc., which may be considered as constant in a particular context
• Locants in chemical-compound names indicating attachments to heteroatoms, e.g. N,N-dimethylaniline
• Stereochemical descriptors as (E) or (Z)

Subscripts:

When, in a given context, different quantities have the same letter symbol or when, for one quantity, different applications or different values are of interest, a distinction can be made by use of subscripts. The following principles for the printing of subscripts apply (see also specific heat capacity $$c_p$$ in "Examples" below).

• A subscript that represents a physical quantity or a mathematical variable, such as a running number, is printed in italic type, e.g. equilibrium constant on a pressure basis $$K_p$$ and equilibrium constant on a concentration basis $$K_c$$.
• Other subscripts, such as those representing words or fixed numbers, are printed in upright type, e.g. Avogadro constant $$N_\mathrm A$$.

There are of course some mixed notations possible:

• Chemical formulae which contain variables, such as the $$n$$ in the general formula for alkanes ($$\ce{C_{n}H_{2n+2}}$$) or $$x$$ in this molecular formula for a superconductor: $$\ce{LaO_{1−x}F_{x}FeAs}$$
• Point groups, for example $$C_n$$, $$S_{2n}$$, $$D_n$$, $$D_{n\mathrm{h}}$$, $$D_{n\mathrm{d}}$$, $$C_{n\mathrm{v}}$$, $$C_{n\mathrm{h}}$$, $$T$$, $$T_\mathrm{h}$$, $$T_\mathrm{d}$$, $$O$$, $$O_\mathrm{h}$$, $$I_\mathrm{h}$$, $$C_{\infty\mathrm{v}}$$, $$D_{\infty\mathrm{h}}$$. More detail can be found in the question on the main site: How are point group character tables typeset correctly?
• The symbol $$\mathrm pK_\mathrm a$$ for the logarithmic acid dissociation constant (read details under "Examples")

# How do I do this?

There are two generic commands that produce roman output, \text{} and \mathrm{}. The main difference between the two commands is the way how math command characters such as the caret ^ or the underline _ are interpreted. In \text{} these get rendered out literally and in \mathrm{} they get interpreted as usual. The use of \mathrm{} is therefore recommended. (The command \rm should be avoided since it is deprecated and only maintained for backwards compatibility.)

The mhchem extension offers two shortcuts, \ce and \pu.

## Display Math and inline Math

There are several possibilities to typeset mathematical formulae.
The inline math mode is invoked by bracing the statement with dollar signs, i.e. $...$.
A displayed mathematical equation, it is centered and typeset a little bigger, can be invoked using double dollar signs as braces, i.e. $$...$$. Here it is also possible to force a line break with \\.
When dealing with more than one equation, an aligning environment should be used instead, i.e. \begin{align}...\end{align}. The alignment character is the &. This can also be used in conjunction with the \ce{...} statements.

# Examples

• Chemical formulae and equations: $$\ce{H2O + HCl <--> H3O+ + Cl-}$$ becomes $$\ce{H2O + HCl <--> H3O+ + Cl-}$$
• Units: $$E = 33.4~\mathrm{kJ\, mol^{-1}}$$ becomes $$E = 33.4~\mathrm{kJ\, mol^{-1}}$$ Note that there is a tilde character between the numbers and the \mathrm{} command, which produces a fixed, non-linebreaking space.
Or: $$E = \pu{123E6 kJ mol-1}$$ becomes $$E = \pu{123E6 kJ mol-1}$$
• Sums and descriptive text: $$m_\text{tot} = \sum_i^N m_i ~\text{for}~N~\text{substances}$$ becomes $$m_\text{tot} = \sum_i^N m_i ~\text{for}~N~\text{substances}$$
• Specific heat capacity $$c_p$$: Here the subscript denotes the constant pressure $$p$$ and is as such written in italics; whereas in the molar heat capacity at constant pressure $$C_{\mathrm m,p}$$, the subscript $$\mathrm m$$ does not represent a quantity but the adjective “molar” and is printed in roman (upright) type.
• In the symbol $$\mathrm pK_\mathrm a$$ for the logarithmic acid dissociation constant, the roman symbol $$\mathrm p$$ is interpreted as an operator ($$\mathrm px=-\lg x$$), the italic symbol $$K$$ represents a quantity (the dissociation constant), and the roman subscript $$\mathrm a$$ represents the word “acid”.
• Locants: O-ethyl hexanethioate; N-methylbenzamide
• Electronic configurations can easily be written using the mathrm command. The MathJaX command \mathrm{(1s)^2 (2s)^2 (2p)^6 (3s)^1} renders as $$\mathrm{(1s)^2 (2s)^2 (2p)^6 (3s)^1}\,.$$ Alternatively you can use \mathrm{[Ne] (3s)^1} to get $$\mathrm{[Ne] (3s)^1}\,.$$
• Stereochemical descriptors are preferably italicized through markdown (*E*) or (*Z*), and not mathjax