Isotope Notation Sometimes the atomic number is left off, because it's always the same for a given element. Thus deuterium could be written 2H. When you're writing out nuclear reactions, though, it's useful to have the atomic number there for reference. If you want to talk, rather than write, about an isotope, you usually use the name of the element plus the mass number: 12C6, or 12C, would be called "carbon 12."

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