Atoms of the same element have the same number of protons, which is called the atomic number. Different isotopes of an element have different numbers of neutrons. As a result, they also have different mass numbers, which is the number of protons plus the number of neutrons. An isotope is usually named by giving the element and the mass number. For example, the symbol for carbon is C and all carbon atoms have 6 protons. The most common isotope of carbon also has 6 neutrons, giving a mass number of 12, and it is written carbon-12 or C. The isotope of carbon that has 8 neutrons is written carbon-14 or C.