and the sum of the protons and neutrons gives the atomic mass, the number of neutrons defines the isotope of that element. For example, most carbon (≈ 99 %) has 6 protons and 6 neutrons and is written ...

When isotopes are part of a larger molecule, we call them isotopologues. For instance, the carbon-12 (12C) isotope contains six protons and six neutrons, but the carbon-13 (13C) isotope is heavier ...