Isotopes are forms of an element that have the same number of protons but different numbers of neutrons. There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium ...

While deuterium H-2, an isotope twice as heavy as hydrogen, is predominantly used in nutrition research, nitrogen-15 is the most common stable isotope used in agriculture. Many other stable isotopes ...

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 ...