Each successively heavier element has one more proton in its nucleus. Uranium, the heaviest element found in sizeable amounts in nature, has 92 protons. Its atomic number is 92. All the isotopes of an element have the same number of protons, but a different number of neutrons. The number of protons plus the number of neutrons equals the mass number of the isotope.
Some elements have many naturally occurring isotopes.
Tin, for example, has 10. The most abundant tin isotope is Sn-120, which makes up about a third of the element. The least abundant is Sn-115. Only 0.34 percent of the tin atoms are this isotope. Except for a few special cases, the relative proportions of the different isotopes in a sample of an element are always the same, no matter where the sample comes from.