The mass number (A) is the number of protons and neutrons present in the nucleus of the atom of a chemical element. There is a notation that allows to represent the atomic and mass number of an element: $^A_ZX$ Knowing the atomic and mass numbers of an element, the number of neutrons that the nucleus of one of its atoms will have can be deduced: neutrons = A - Z

Atoms of an element (same atomic number) that have different mass numbers are called isotopes . Thus, hydrogen has three isotopes. Hydrogen has a proton in the nucleus, no neutrons (atomic number 1, mass number 1). Deuterium has one proton and one neutron (atomic number 1, mass number 2). The third isotope of hydrogen is called tritium, it has one proton and two neutrons (atomic number 1, mass number 3). All isotopes of an element have the same atomic number and differ in mass number.

If you look at uranium, two of its most common isotopes are uranium-268 and uranium-238. Uranium has atomic number 92, uranium-235 has 92 protons and 43 neutrons in its nucleus. Uranium-238 has 92 protons and 46 neutrons.

The isotopes of an element have similar chemical properties as they contain the same number of electrons (responsible for the chemical behavior of the atom).