When the nucleus of an atom does not hold together well, it loses energy and mass which are emitted from the atom as radiation. Known as radioactive decay, this phenomenon occurs in certain atoms, which are said to be unstable. The three most common types of radioactive decay are: beta decay, alpha decay, and gamma decay.
The electromagnetic force works to push the positively-charged protons away from one another. However, another force, the strong force, tends to pull subatomic particles together. Since neutrons have no electrical charge and yet, like protons, are acted upon by the strong force, they act as a kind of glue inside an atom’s nucleus. Having not enough glue, or too much glue, makes an atom unstable, so that it undergoes a process called radioactive decay. The number of protons in an atom’s nucleus defines which element it is, but for each element the number of neutrons can vary. Known as isotopes, these differing forms of each element are distinguished by a number written as a superscript to the left of the atomic symbol (indicating the total number of protons and neutrons), and vary in their stability. For some elements, just one isotope is stable, for some two or more isotopes are stable, while for many of the heavy elements none of their isotopes are stable. For example, hydrogen occurs as three isotopes: protium (1H), whose nucleus consists of just a proton, deuterium (2H), whose nucleus consists of a proton and one neutron, and tritium (3H), whose nucleus consists of a proton and two neutrons. 1H and 2H are stable, while tritium decays in a process known as beta decay to an isotope of helium, called helium-3 (3He, consisting of two protons and one neutron), though most helium found on Earth is of another isotope, 4He, containing two neutrons.
Specifically, the radioactive decay occurring in tritium is known as beta- decay. Essentially what happens is that one of the two neutrons converts to a proton, in the process creating an electron, which is emitted from the atom as a negatively-charged beta particle (plus a particle known as an electron antineutrino). Stable isotopes of carbon are 12C, with six neutrons, and 13C, with seven neutrons, while 14C (eight neutrons) and 11C (five neutrons) are unstable. Like tritium, 14C undergoes beta- decay; this results in the conversion of 14C to a nitrogen isotope, 14N. In contrast 11C, undergoes a process called beta+ decay, resulting in boron-11 (11B). Unlike beta- decay, beta+ yields a positively-charged beta particle known as a positron (plus a particle known as an electron neutrino). Another example of beta+ decay is fluorine-18 (18F), which decays to oxygen-18 (18O). It’s important to know about 18F beta+ decay, as this is the most common isotope used in positron emission tomography (PET) scanning, a vital imaging modality in medicine. Similarly, phosphorus-32, phosphorus-33, and sulfur-35, all of which undergo beta- decay, are used extensively in biology research. Beta particles also are called beta radiation.
Another type of radioactive decay is characterized by the emission from an atomic nucleus of a particle much larger than a beta particle. It can be a proton or neutron, or a cluster protons and neutrons. Mostly commonly, it is a cluster known as an alpha particle, also called alpha radiation, consisting of two protons and two neutrons, which is to say a nucleus of 4He. This form of decay occurs in certain very large atoms such as uranium-238 (238U) , radium-226, and radon-222. Finally, there is a type of radioactivity characterized by a transfer of energy from the nucleus, not as a beta particle or a larger particle, but as high-energy electromagnetic radiation. The most common subtype of this category is gamma decay, characterized by a nucleus releasing a very high energy photon (particle of electromagnetic radiation) known as a gamma ray, or gamma radiation.
Whether decaying through beta, alpha, or gamma decay, each radioactive isotope has a specific rate by which it decay into something else. This rate of decay is defined mathematically by the half-life, the amount of time that it takes for half of the atoms of a sample of an unstable element to decay to a different element. The duration of the half-life varies enormously from isotope to isotope, and often the product is itself unstable and thus subsequently decays into yet something else.