Different atoms of the same element can have different numbers of neutrons. Atoms having varying numbers of neutrons are known as Isotopes. Different isotopes of an element are identified by their atomic mass number. The atomic mass number is simply the total of all the protons and neutrons present in a nucleus. For example, uranium 238 has a mass number of 238, which is the total of its 92 protons and 146 neutrons.
Many isotopes contain combinations of neutrons and protons that are unstable.
These isotopes will always move toward a stable condition. They do this in a number of ways. Depending on how unstable the isotopes are, they may undergo radioactive decay. Radioactive decay occurs when isotopes move toward a stable condition by releasing particles. Both alpha and beta decay change the number of protons in a nucleus resulting in a new daughter element.