One of the ways in which isotopes of an element differ from each other is in their lifetime. Most of those which occur naturally are stable and os appear to last for ever. This is because the number of neutrons is sufficient to counterbalance the repulsive force between the protons due to their positive charges. In a few isotopes this balance is not achieved and so they can change into different nuclides in a process known as radioactive decay. This normally involves the emission of one or more particles from the nucleus of the atom and results in the formation of a more stable nuclide.