The answer is isotopes. Isotopes are different kind of atoms of the same element. Elements are identified by the number of protons in the nucleus (atomic number), so all the aoms of uranium have the same number of protons. When one element can have atoms with different number of neutrons (which is pretty normal) the different forms are called isotopes. So, isotopes are different versions of the same element which differentiate from each other in the number of neutrons.