nucleons
and Z, the atomic number, is the number of protons. The protons and neutrons that make up the nucleus of an atom are callednucleons, and an atom with a particular number of protons and neutrons is called anuclide. Nuclides with the same number of protons but different numbers of neutrons are called isotopes. Isotopes can also be represented by an alternative notation that uses the name of the element followed by the mass number, such as carbon-12. The stable isotopes of oxygen, for example, can be represented in any of the following ways:.