Deuterium is often represented by the chemical symbol D. Since it is an isotope of hydrogen with mass number 2, it is also represented by H. IUPAC allows both D and H, though H is preferred. A distinct chemical symbol is used for convenience because of the isotopes common use in various scientific processes. Also, its large mass difference with protium (H) confers non-negligible chemical differences with protium compounds. Deuterium has a mass of , about twice the mean hydrogen atomic weight of , or twice protiums mass of . The isotope weight ratios within other chemical elements are largely insignificant in this regard.