Editing Nuclear physics (section)

===Nuclear decay===
{{Main|Radioactivity|Valley of stability}}
Eighty elements have at least one [[stable isotope]] which is never observed to decay, amounting to a total of about 251 stable nuclides. However, thousands of [[isotope]]s have been characterized as unstable. These "radioisotopes" decay over time scales ranging from fractions of a second to trillions of years. Plotted on a chart as a function of atomic and neutron numbers, the binding energy of the nuclides forms what is known as the [[valley of stability]]. Stable nuclides lie along the bottom of this energy valley, while increasingly unstable nuclides lie up the valley walls, that is, have weaker binding energy.

The most stable nuclei fall within certain ranges or balances of composition of neutrons and protons: too few or too many neutrons (in relation to the number of protons) will cause it to decay. For example, in [[beta decay]], a [[nitrogen]]-16 atom (7 protons, 9 neutrons) is converted to an [[oxygen]]-16 atom (8 protons, 8 neutrons)<ref>Not a typical example as it results in a "doubly magic" nucleus</ref> within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is converted by the [[weak interaction]] into a proton, an electron and an [[antineutrino]]. The element is transmuted to another element, with a different number of protons.

In [[alpha decay]], which typically occurs in the heaviest nuclei, the radioactive element decays by emitting a helium nucleus (2 protons and 2 neutrons), giving another element, plus [[helium-4]]. In many cases this process continues through [[decay chain|several steps]] of this kind, including other types of decays (usually beta decay) until a stable element is formed.

In [[gamma decay]], a nucleus decays from an excited state into a lower energy state, by emitting a [[gamma ray]]. The element is not changed to another element in the process (no [[nuclear transmutation]] is involved).

Other more exotic decays are possible (see the first main article). For example, in [[internal conversion]] decay, the energy from an excited nucleus may eject one of the inner orbital electrons from the atom, in a process which produces high speed electrons but is not beta decay and (unlike beta decay) does not transmute one element to another.