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 ${}_Z^{A}X$, $Z$ 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.