Radioactive decay occurs when an atomic nucleus of an unstable atom looses energy by emitting ionizing particles. There are many different types of radioactive decay. The result of decay is either that the nucleus enters a different state or that the number of nucleons (protons and/or neutrons) in the nucleus changes. The first types of decay processes that were discovered were alpha decay, beta decay and gamma decay.
During alpha decay, the nucleus ejects what is called an alpha particle or helium nucleus. This particle contains two protons and two neutrons. It is the most common type of decay where the number of nucleon changes. (More rare types of nucleon emission include protons, neutrons or nuclei of elements other than helium.)
Beta decay consists of a process where a nucleus emits an electron or a positron (the anti-matter version of the electron) and a neutrino (an almost massless particle.) This results in the conversion of a proton to a neutron or a neutron to a proton. (Orbital electrons can be captured which converts a proton into a neutron which is called electron capture.)
Both alpha and beta decay change the nucleus into a different element. This is called atomic transmutation.
Gamma decay does not transmute one element to another. Energy is carried away from the nucleus as a gamma ray which is a highly energetic photon or particle of energy. (An orbital electron can also be ejected from the electron cloud around the nucleus which is referred to as internal conversion.)
A nucleus sometimes emits a neutron which results in a change from one isotope of the element to another isotope of the same element.
Another type of radioactive decay that does not result in a well defined outcome is called spontaneous fission. This occurs when a large unstable nucleus breaks into two or three smaller nuclei. This is usually accompanied by emission of gamma rays, neutrons or other particles.
So to recap, when a nucleus undergoes radioactive decay, the result can be the emission of an electron, a neutron, a proton, a gamma ray, a neutrino and/or one or more nuclei of elements with lower atomic numbers.
At the level of an individual atomic nucleus, the time at which decay will occur is strictly statistical. That is to say, we don't know when any particular nucleus in a group consisting of a specific isotope of a particular element will decay but the number of such nuclei that will decay in a specific period of time is always the same. For any given elemental isotope, this rate is predicted from the measured decay constant for that isotope. The time it takes for half the atoms in a specified number of identical atoms to decay is referred to as the half-life for that isotope. The half-life is a very important property of a particular isotope and one that we will be discussing further in future articles.
The danger of radioactive decay lies in the effect of the emissions of decaying nuclei on living systems.