Editing Mass number (section)

{{short description|Number of heavy particles in the atomic nucleus}}
{{distinguish|Atomic number|Atomic mass|Relative atomic mass}}
{{Nuclear physics|expanded=Nuclides' classification}}

The '''mass number''' (symbol ''A'', from the German word: ''Atomgewicht'', "atomic weight"),<ref>[[Jensen, William B.]] (2005). The Origins of the Symbols A and Z for Atomic Weight and Number. ''J. Chem. Educ.'' 82: 1764. [http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/123.%20A%20&%20Z.pdf link].</ref> also called  '''atomic mass number''' or '''nucleon number''', is the total number of [[proton]]s and [[neutron]]s (together known as [[nucleon]]s) in an [[atomic nucleus]]. It is approximately equal to the [[atomic mass|''atomic'' (also known as ''isotopic'') mass]] of the [[atom]] expressed in [[atomic mass unit]]s. Since protons and neutrons are both [[baryon]]s, the mass number ''A'' is identical with the [[baryon number]] ''B'' of the nucleus (and also of the whole atom or [[ion]]). The mass number is different for each [[isotope]] of a given [[chemical element]], and the difference between the mass number and the [[atomic number]]&nbsp;''Z'' gives the [[Neutron number|number of neutrons]] (''N'') in the nucleus: {{nowrap|1=''N'' = ''A'' − ''Z''}}.<ref>{{cite web|url=http://education.jlab.org/qa/pen_number.html|title=How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon, silver, gold, etc...?|publisher=Thomas Jefferson National Accelerator Facility|access-date=2008-08-27}}</ref>

The mass number is written either after the element name or as a [[superscript]] to the left of an element's symbol. For example, the most common isotope of [[carbon]] is [[carbon-12]], or {{SimpleNuclide|carbon|12}}, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (''Z'') as a subscript to the left of the element symbol directly below the mass number: {{nuclide|carbon|12}}.<ref>{{cite web|url=http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson35.htm|title=Elemental Notation and Isotopes|publisher=Science Help Online|access-date=2008-08-27|url-status=dead|archive-url=https://web.archive.org/web/20080913063710/http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson35.htm|archive-date=2008-09-13}}</ref>