Editing Atom (section)

=== Mass ===
{{Main|Atomic mass|mass number}}

The large majority of an atom's mass comes from the protons and neutrons that make it up. The total number of these particles (called "nucleons") in a given atom is called the [[mass number]]. It is a positive integer and dimensionless (instead of having dimension of mass), because it expresses a count. An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons).

The actual [[Invariant mass|mass of an atom at rest]] is often expressed in [[dalton (unit)|daltons]] (Da), also called the unified atomic mass unit (u). This unit is defined as a twelfth of the mass of a free neutral atom of [[carbon-12]], which is approximately {{val|1.66|e=-27|u=kg}}.<ref name=iupac /> [[hydrogen atom|Hydrogen-1]] (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825&nbsp;Da.<ref name=chieh2001 /> The value of this number is called the [[atomic mass]]. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly 14&nbsp;Da), but this number will not be exactly an integer except (by definition) in the case of carbon-12.<ref name=nist_wc /> The heaviest [[stable atom]] is lead-208,<ref name=s131 /> with a mass of {{val|207.9766521|u=Da}}.<ref name=audi2003 />

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of [[Mole (unit)|moles]]. One mole of atoms of any element always has the same number of atoms (about [[Avogadro constant|{{val|6.022|e=23}}]]). This number was chosen so that if an element has an atomic mass of 1&nbsp;u, a mole of atoms of that element has a mass close to one gram. Because of the definition of the [[Atomic mass unit|unified atomic mass unit]], each carbon-12 atom has an atomic mass of exactly 12&nbsp;Da, and so a mole of carbon-12 atoms weighs exactly 0.012&nbsp;kg.<ref name=iupac>{{cite book
|last=Mills
|first=Ian
|author2=Cvitaš, Tomislav
|author3=Homann, Klaus
|author4=Kallay, Nikola
|author5=Kuchitsu, Kozo
|title=Quantities, Units and Symbols in Physical Chemistry
|publisher=[[International Union of Pure and Applied Chemistry]], Commission on Physiochemical Symbols Terminology and Units, Blackwell Scientific Publications
|location=Oxford
|edition=2nd
|year=1993
|isbn=978-0-632-03583-0
|oclc=27011505
|url=https://archive.org/details/quantitiesunitss0000unse/page/70
|page=[https://archive.org/details/quantitiesunitss0000unse/page/70 70]
}}</ref>