For lighter elements the number of protons and neutrons are approximately equal. For example, the most common isotope of helium with two protons is ⁴He, so it has 4 - 2 = 2 neutrons. Oxygen, with eight protons, has eight neutrons in its most common isotope, ¹⁶O.

In elements heavier than calcium (20 protons, 20 neutrons) the number of neutrons is larger than the number of protons. In uranium-238 (92 protons, 146 neutrons) the ratio is almost 3 neutrons for 2 protons.