A radioactive atom emits particles and radiation from its nucleus because the nucleus is unstable. This is called radioactive decay and isotopes which undergo this process are called radioisotopes or radionuclides. Nearly all radionuclides have a relatively high ratio of neutrons to protons, for example, uranium with 143 neutrons to 92 protons. Elements with atomic numbers greater than 92 do not occur naturally but may be prepared artificially by bombardment with neutrons or protons. These elements are all radioactive and many are important in a wide variety of uses. Some elements lighter than 92 also do not occur naturally, because all their isotopes are radioactive and have therefore long since decayed. Examples are technetium and astatine.
Because these changes affect the nucleus and can change the number of protons, unlike chemical reactions, the products of nuclear reactions can be new elements. Radioactive decay can involve a series of steps called a decay series. The first, unstable radionuclide is called the 'parent' isotope and all the stable isotopes that eventually result are called 'daughters'. Important types of radioactive decay are described below.
Alpha (⍺) decay occurs when an alpha particle or helium-4 nucleus (42He2+) is emitted. Therefore, alpha particles carry a positive charge. Alpha particles are the largest decay product, and their loss decreases an atom's atomic number by 2 and its mass by 4. As an example, the balanced nuclear equation below shows uranium-234 undergoing alpha decay to thorium-230:
23492U →42He + 23090Th
Beta (β) decay occurs when a beta minus particle, the equivalent of a high speed electron (0-1e or 0-1β) is emitted. A positron is the antimatter equivalent of an electron. A neutron transforms into a proton, an electron and a third particle with no charge (an antineutrino) to ensure conservation of energy and momentum. In beta decay, the proton remains behind in the nucleus. The beta particle carries a negative charge, while the proton increases the atomic number by 1. The overall mass of the atom is approximately unchanged. As an example, the equation below represents carbon-14 undergoing beta decay into nitrogen-14:
146C → 0-1β +147N
Positron emission (also called beta positive or beta plus decay) occurs when the nucleus emits a positron () a particle that has the same mass and size as an electron but has a positive charge. This process occurs when the neutron-to-proton ratio is too small for stability. A proton transforms into a neutron (p →10n + 0+1e, decreasing the proton number by 1), a positron and a third particle of no mass or charge (a neutrino) to ensure conservation of energy and momentum. After the positron is emitted, the neutron remains behind in the nucleus, so the overall mass of the nucleus is unchanged.
Gamma (𝛾) decay consists of the emission of electromagnetic radiation of very high frequency (short wavelength) and energy. Its symbol, 00𝛾, indicates its emission does not affect charge or mass, and for this reason, gamma radiation is often not included in nuclear equations. Gamma radiation is emitted from excited nuclei of elements that have formed as a result of radioactive decay by emitting alpha or beta particles, and removes further energy from their nucleus. Gamma ray photons have energies of about 1 x 10-12 joules.