section_path: Nomenclature and symbols/Chemical symbols/Isotope symbols
section_name: Isotope symbols
paragraph_text: Isotope symbols
  
  Isotopes of an element are distinguished by mass number (total protons and neutrons), with this number combined with the element's symbol. IUPAC prefers that isotope symbols be written in superscript notation when practical, for example C and U. However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.
  
  As a special case, the three naturally occurring isotopes of hydrogen are often specified as H for H (protium), D for H (deuterium), and T for H (tritium). This convention is easier to use in chemical equations, replacing the need to write out the mass number each time. Thus, the formula for heavy water may be written D O instead of H O.