Editing Atom (section)

=== Radioactive decay ===
{{Main|Radioactive decay}}

[[File:Isotopes and half-life.svg|right|thumb|This diagram shows the [[half-life]] (T<sub>{{frac|1|2}}</sub>) of various isotopes with Z protons and N neutrons.]]

Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1&nbsp;fm.<ref name=splung />

The most common forms of radioactive decay are:<ref>{{cite book
|last=L'Annunziata<!-- Note: the single quote mark before the name is correct. -->
|first=Michael F.
|year=2003|title=Handbook of Radioactivity Analysis
|url=https://archive.org/details/handbookradioact00lann|url-access=limited|publisher=Academic Press|isbn=978-0-12-436603-9
|oclc=16212955|pages=[https://archive.org/details/handbookradioact00lann/page/n22 3]–56}}</ref><ref name=firestone20000522 />
* [[Alpha decay]]: this process is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower [[atomic number]].
* [[Beta decay]] (and [[electron capture]]): these processes are regulated by the [[weak force]], and result from a transformation of a neutron into a proton, or a proton into a neutron. The neutron to proton transition is accompanied by the emission of an electron and an [[antineutrino]], while proton to neutron transition (except in electron capture) causes the emission of a [[positron]] and a [[neutrino]]. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one. Electron capture is more common than positron emission, because it requires less energy. In this type of decay, an electron is absorbed by the nucleus, rather than a positron emitted from the nucleus. A neutrino is still emitted in this process, and a proton changes to a neutron.
* [[Gamma decay]]: this process results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. The excited state of a nucleus which results in gamma emission usually occurs following the emission of an alpha or a beta particle. Thus, gamma decay usually follows alpha or beta decay.

Other more rare types of [[radioactive decay]] include ejection of neutrons or protons or clusters of [[nucleon]]s from a nucleus, or more than one [[beta particle]]. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is [[internal conversion]]—a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called [[Spontaneous fission|spontaneous nuclear fission]].

Each [[radioactive isotope]] has a characteristic decay time period—the [[half-life]]—that is determined by the amount of time needed for half of a sample to decay. This is an [[exponential decay]] process that steadily decreases the proportion of the remaining isotope by 50% every half-life. Hence after two half-lives have passed only 25% of the isotope is present, and so forth.<ref name=splung />