Editing Mass number (section)

==Relative atomic mass of an element==

The mass number should also not be confused with the [[standard atomic weight]] (also called [[atomic weight]]) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the [[atomic mass constant]].<ref>{{cite book |doi=10.1351/goldbook.R05258 |doi-access=free |chapter=Relative atomic mass (Atomic weight), A<sub>r</sub> |title=The IUPAC Compendium of Chemical Terminology |year=2014 }}
</ref> The atomic weight is a ''mass'' ratio, while the mass number is a ''counted'' number (and so an integer). 

This weighted average can be quite different from the near-integer values for individual isotopic masses. For instance, there are two main [[isotopes of chlorine]]: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subjected to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/[[Mole (unit)|mol]]).

Moreover, the weighted average mass can be near-integer, but at the same time not corresponding to the mass of any natural isotope. For example, [[bromine]] has only two stable isotopes, <sup>79</sup>Br and <sup>81</sup>Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol),<ref>{{cite web|url=http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl|title=Atomic Weights and Isotopic Compositions for All Elements|publisher=NIST}}</ref> even though the [[Isotopes of bromine|isotope <sup>80</sup>Br]] with such mass is unstable.