= Chemical symbol
A <chemical symbol> is the standardized letter label for a <chemical element>, such as $\mathrm C$ for <carbon> or $\mathrm{Co}$ for <cobalt>. Capitalization is meaningful: $\mathrm{Co}$ names one element, whereas $\mathrm{CO}$ is a formula containing <carbon> and <oxygen>.
Nuclide notation adds information to the element symbol: in ${}^{A}_{Z}X$, $Z$ is the <atomic number> and $A$ the <mass number>. An <isotope> changes $A$ without changing the element identity fixed by $Z$. Ionic charge is written separately, as in $\mathrm{Fe}^{3+}$. A formula's subscripts count atoms per formula unit; they are neither atomic numbers nor charges.
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