Absolute dating requires assigning a specific age to an event or object that is typically measured in years before the present. Although there are a number of techniques that can be used to determine an absolute age the most common techniques utilize the decay rates of unstable isotopes and is called radiometric dating.
An isotope is a variation of an individual element that contains different numbers of neutrons. For example the element hydrogen contains exactly 1 proton and has a mass of 1, isotopes of hydrogen also only contain 1 proton but may contain either 1 or 2 neutrons and therefore have masses of either 2 or 3 amu. How many protons and neutrons contained in the nucleus determines the isotopes stability. Much like the electrons orbiting the nucleus, protons and neutrons are arranged in a very specific pattern within the nucleus that minimizes electronic and quantum mechanical forces. As a result of this pattern certain numbers of protons and neutrons are much more stable than others. If the nucleus of an atom contains an unstable number of protons or neutrons it may decay, or give off subatomic particles, in order to reach a stable number of protons or neutrons. During this process enormous amounts of energy are also released in the form of heat. In nature it is this process that is responsible for a majority of Earth's natural heat. Power plants also utilize the energy from radioactive decay to turn water to the steam that turns turbines in nuclear power plants.
Unstable isotopes can decay through three different processes; alpha decay, beta decay, and electron capture decay. In alpha decay an unstable nucleus ejects two protons and two neutrons--referred to as an alpha particle. As a result of loosing two protons the atomic number is decreased by two and the original isotope, known as the parent isotope, is transformed into a new element referred to as the daughter isotope. Since an alpha particle is composed of two protons and two neutrons the atomic mass of the daughter isotope is 4 amu less than the parent isotope.
Protons and neutrons are themselves composed of even smaller subatomic particles and can also decay. In beta decay a neutron emits a very fast moving electron, called a beta particle. As a result of loosing a negatively charged particle the neutron is then transformed into a proton. As a result of beta decay the atomic number of the isotope is increased by 1 thereby transforming it into another element. Since the emitted electron is essentially massless and protons and neutrons have the same mass the atomic mass of the daughter isotope is the same as the parent isotope.
In electron capture decay a proton absorbs a very fast moving electron and is transformed into a neutron. As a result the atomic number is decreased by 1 and the atomic mass of the daughter isotope is the same as the parent isotope.
Unstable, or radiioactive, isotopes may decay in one step or through a series of decay steps. For example, Rubidium 87 decays to Strontium 87 by a single beta emission whereas Uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps.