The number of neutrons in an atom’s nucleus approximates the number of protons but it may vary somewhat. Keep in mind that an element’s chemical nature is given by the number of protons and, hence, electrons. A differing number of neutrons has no effect on the element’s chemical nature; it only effects mass. Atoms of an element with different numbers of neutrons are called isotopes. Thus, isotopes of an element have the same number of protons and electrons, but differing numbers of neutrons.
The number of protons in an atom (atomic number) plus the number of neutrons is that atom’s atomic weight. The following link provides a table of stable isotopes of all the elements. For example, there are three isotopes of hydrogen. Most atoms of hydrogen consist of a proton by itself and a single electron. However some atoms of hydrogen have one and some have two neutrons in addition to the proton. Thus, there are three isotopes of hydrogen.