In science, a subscript is a number, character or symbol positioned next to and a little below another character. In chemistry, subscripts on the left are used to display the atomic number of an element, indicating the number of protons in an isotope.
Subscripts on the right of an elemental symbol indicate the number of atoms in a molecule for a particular nuclide. Unlike a subscript, a superscript is positioned next to and above another character. Superscripts are used to denote the mass number of an element in chemistry, which is determined by the sum of protons and neutrons. When there is no subscript beside an elemental symbol, it indicates there is one atom present, as the subscript 1 is never shown.
A subscript can also be used at the end of a parenthesis in a chemical formula, indicating that the enclosed group should be multiplied by the number represented on the subscript. When using subscripts in scientific text, it is important to adhere to the style conventions developed by the International System of Units. Subscripts should be italicized when used to represent variables, running numbers and quantities. If the subscript is descriptive, it should be left in roman type with no style modifications. The same rules apply when using superscripts in scientific documents.