Chemical symbols are the one or two-letter abbreviations for an element in the periodic table, such as C for carbon, H for hydrogen, O for oxygen, or Co for cobalt. The first letter of a chemical symbol is always capitalized. If the symbol has two letters, the second letter is written in lowercase to avoid mistaking it for a diatomic molecule written using the same letters. Most chemical symbols are based on the first or the first two letters of the element's English name, while some symbols are derived from their Latin or Greek name. Fe for iron comes from the Latin name Ferrum, and sodium is denoted as Na for its Latin name Natrium. Likewise, Aurum is the Latin name for gold, so its symbol is Au. A chemical symbol can include additional information about that element, such as its atomic number, usually written as a left subscript, and its mass number, generally written as a left superscript.