A variant of a basic element is called an isotope. An isotope is an element composed of atoms that contain a different number of neutrons compared to the basic element. Excluding hydrogen, all of the atomic nucleus in matter is comprised of both protons and neutrons. The only difference is how many each will have. Usually, the amount of protons and neutrons will be the same. But in an isotope, this does not hold true. An example will be uranium. The most common kind of this radioactive element is 238U, but it has three more neutrons than 235U, which is the kind of uranium used for nuclear weapons.
An element that does not have the necessary neutrons will make the nucleus unstable. Protons are positively charged particles, which mean they will repel each other. Neutrons in the nucleus play an important role in that it separates the protons slightly. This makes the whole configuration stable. When this configuration becomes unstable nuclear decay happens. This results in the atom emitting radioactive particles. Half-life is the term used for the rate at which an isotope undergoes radioactive decay. The half-life is the interval after which one half of the radioactive element breaks down. The half-life of elements also differs. Some have a half-life of just fractions of a second, while others have a half-life so long it’s several times the age of the universe. Of course, there are also some isotopes that do not have a half-life because these are not radioactive. An example of this is Helium-3.