Editing Atom (section)

=== Subatomic particles ===
{{Main|Subatomic particle}}
Though the word ''atom'' originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various [[subatomic particle]]s. The constituent particles of an atom are the [[electron]], the [[proton]], and the [[neutron]].

The electron is the least massive of these particles by four orders of magnitude at {{val|9.11|e=-31|u=kg}}, with a negative [[Electric charge|electrical charge]] and a size that is too small to be measured using available techniques.<ref>{{cite book|last=Demtröder|first=Wolfgang|year=2002|title=Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics|url=https://archive.org/details/atomsmoleculesph00demt_277|url-access=limited|publisher=Springer|edition=1st|isbn=978-3-540-20631-6|oclc=181435713|pages=[https://archive.org/details/atomsmoleculesph00demt_277/page/n51 39]–42}}</ref> It was the lightest particle with a positive rest mass measured, until the discovery of [[neutrino]] mass. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an [[ion]]. Electrons have been known since the late 19th century, mostly thanks to [[J.J. Thomson]]; see [[history of subatomic physics]] for details.

Protons have a positive charge and a mass of {{val|1.6726|e=-27|u=kg}}. The number of protons in an atom is called its [[atomic number]]. [[Ernest Rutherford]] (1919) observed that nitrogen under alpha-particle bombardment ejects what appeared to be hydrogen nuclei. By 1920 he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it [[proton]].

Neutrons have no electrical charge and have a mass of {{val|1.6749|e=-27|u=kg}}.<ref>{{cite book|last=Woan|first=Graham|year=2000|title=The Cambridge Handbook of Physics|publisher=Cambridge University Press|isbn=978-0-521-57507-2|oclc=224032426|page=[https://archive.org/details/cambridgehandboo0000woan/page/8 8]|url=https://archive.org/details/cambridgehandboo0000woan/page/8}}</ref><ref name="2014 CODATA">Mohr, P.J.; Taylor, B.N. and Newell, D.B. (2014), [http://physics.nist.gov/constants "The 2014 CODATA Recommended Values of the Fundamental Physical Constants"] {{Webarchive|url=https://web.archive.org/web/20120211083747/http://physics.nist.gov/cuu/Constants/index.html |date=11 February 2012 }} (Web Version 7.0). The database was developed by J. Baker, M. Douma, and [[Svetlana Kotochigova|S. Kotochigova]]. (2014). National Institute of Standards and Technology, Gaithersburg, Maryland 20899.</ref> Neutrons are the heaviest of the three constituent particles, but their mass can be reduced by the [[nuclear binding energy]]. Neutrons and protons (collectively known as [[nucleon]]s) have comparable dimensions—on the order of {{val|2.5|e=-15|u=m}}—although the 'surface' of these particles is not sharply defined.<ref>{{cite book|last=MacGregor|first=Malcolm H.|year=1992|title=The Enigmatic Electron|publisher=Oxford University Press|isbn=978-0-19-521833-6|oclc=223372888|pages=[https://archive.org/details/astronomyencyclo0000unse/page/33 33–37]|url=https://archive.org/details/astronomyencyclo0000unse/page/33}}</ref> The neutron was discovered in 1932 by the English physicist [[James Chadwick]].

In the [[Standard Model]] of physics, electrons are truly elementary particles with no internal structure, whereas protons and neutrons are composite particles composed of [[elementary particle]]s called [[quark]]s. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two [[up quark]]s (each with charge +{{sfrac|2|3}}) and one [[down quark]] (with a charge of −{{sfrac|1|3}}). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles.<ref name=pdg2002 /><ref name=schombert2006 />

The quarks are held together by the [[strong interaction]] (or strong force), which is mediated by [[gluon]]s. The protons and neutrons, in turn, are held to each other in the nucleus by the [[nuclear force]], which is a residuum of the strong force that has somewhat different range-properties (see the article on the nuclear force for more). The gluon is a member of the family of [[gauge boson]]s, which are elementary particles that mediate physical forces.<ref name=pdg2002 /><ref name=schombert2006 />