Editing Atom (section)

=== Nuclear properties ===
{{Main|Isotope|Stable isotope|List of nuclides|List of elements by stability of isotopes}}
By definition, any two atoms with an identical number of ''protons'' in their nuclei belong to the same [[chemical element]]. Atoms with equal numbers of protons but a different number of ''neutrons'' are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons ([[hydrogen-1]], by far the most common form,<ref name=matis2000 /> also called protium), one neutron ([[deuterium]]), two neutrons ([[tritium]]) and [[isotopes of hydrogen|more than two neutrons]]. The known elements form a set of atomic numbers, from the single-proton element [[hydrogen]] up to the 118-proton element [[oganesson]].<ref name=weiss20061017 /> All known isotopes of elements with atomic numbers greater than 82 are radioactive, although the radioactivity of element 83 ([[bismuth]]) is so slight as to be practically negligible.<ref name=s131>{{cite book|last=Sills|first=Alan D.|year=2003|title=Earth Science the Easy Way|publisher=Barron's Educational Series|isbn=978-0-7641-2146-3|oclc=51543743|pages=[https://archive.org/details/earthscienceeasy00alan/page/131 131–134]|url=https://archive.org/details/earthscienceeasy00alan/page/131}}</ref><ref name=dume20030423 />

About 339 nuclides occur naturally on [[Earth]],<ref name=lidsay20000730 /> of which 251 (about 74%) have not been observed to decay, and are referred to as "[[stable isotope]]s". Only 90 nuclides are stable [[list of nuclides|theoretically]], while another 161 (bringing the total to 251) have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 35 radioactive nuclides have half-lives longer than 100 million years, and are long-lived enough to have been present since the birth of the [[Solar System]]. This collection of 286 nuclides are known as [[primordial nuclide]]s. Finally, an additional 53 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay (such as [[radium]] from [[uranium]]), or as products of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14).<ref name=tuli2005 /><ref group=note>For more recent updates see [[Brookhaven National Laboratory]]'s [http://www.nndc.bnl.gov/chart Interactive Chart of Nuclides] ] {{Webarchive|url=https://web.archive.org/web/20200725182342/https://www.nndc.bnl.gov/nudat2/ |date=25 July 2020 }}.</ref><!-- See article [[list of nuclides]]. The numbers are derived by [[WP:CALC]] (counting the table), which is not [[WP:OR]]-->

For 80 of the chemical elements, at least one [[stable isotope]] exists. As a rule, there is only a handful of stable isotopes for each of these elements, the average being 3.1 stable isotopes per element. Twenty-six "[[monoisotopic element]]s" have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element [[tin]]. Elements [[technetium|43]], [[promethium|61]], and all elements numbered [[bismuth|83]] or higher have no stable isotopes.<ref name=CRC>CRC Handbook (2002).</ref>{{rp|1–12}}

Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the [[Nuclear shell model|shell model]] of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 251 known stable nuclides, only four have both an odd number of protons ''and'' odd number of neutrons: [[hydrogen-2]] ([[deuterium]]), [[lithium-6]], [[boron-10]], and [[nitrogen-14]]. ([[Tantalum-180m]] is odd-odd and observationally stable, but is predicted to decay with a very long half-life.) Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: [[potassium-40]], [[vanadium-50]], [[lanthanum-138]], and [[lutetium-176]]. Most odd-odd nuclei are highly unstable with respect to [[beta decay]], because the decay products are even-even, and are therefore more strongly bound, due to [[Semi-empirical mass formula#Pairing term|nuclear pairing effects]].<ref>{{cite book |last=Krane |first=K. |year=1988 |title=Introductory Nuclear Physics |url=https://archive.org/details/introductorynucl00kran |url-access=limited |publisher=[[John Wiley & Sons]] |isbn=978-0-471-85914-7 |pages=[https://archive.org/details/introductorynucl00kran/page/n90 68]}}</ref>