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In [[chemistry]] and [[physics]],  '''atomic mass''' (formerly known as '''atomic weight''') is the [[mass]] of an atom expressed in [[unified atomic mass unit]]s (u). Atomic mass is numerically equal to '''relative atomic mass''', denoted by ''A''<sub>r</sub>( X), where X is the [[isotope]] of which the mass is indicated. The relative atomic mass is the ratio of atomic mass to one twelfth of the mass of the [[nuclide]] <sup>12</sup>C at rest in its nuclear and electronic ground state.

The difference between atomic mass and relative atomic mass is that the former has a dimension (u), while the latter is dimensionless.

==Handling of isotopic masses==
Different [[isotopes]] of an element have different numbers of neutrons and the same number (the [[atomic number]] ''Z'') of protons. So, different isotopes of a given element have the same charge but differ in mass. For example, the element [[carbon]] (atomic number ''Z'' = 6, i.e., 6 protons) has two stable isotopes and one radioactive&mdash;but long-lived&mdash;isotope. The respective  atomic masses are: <sup>12</sup>C: 12 u (six neutrons),  <sup>13</sup>C: 13.0033548378 u  (seven neutrons), and <sup>14</sup>C: 14.003241988 u (eight neutrons). The ''relative'' atomic mass of those three isotopes are, respectively, the dimensionless numbers 12, 13.0033548378, and 14.003241988.

In practice there are two ways of dealing with the different masses of [[isotope]]s:
#In [[high resolution spectroscopy]] and [[mass spectrometry]] masses of isotopes are observed in the spectra. That is, one can distinguish the spectral peaks arising from the different  [[isotopologue]]s, (same molecule, different isotopic composition) in the sample. In these fields it is common to consider the samples as mixtures of of different isotopologues, in much the same way as when the sample consists of different compounds. Hydrogen chloride,  for instance, would be seen as a mixture of  the following isotopologues: H&ndash;<sup>35</sup>Cl, D&ndash;<sup>35</sup>Cl,  H&ndash;<sup>37</sup>Cl, and D&ndash;<sup>37</sup>Cl.
#In most of practical [[chemistry]] different isotopologues&mdash;always present in  "off-the-shelf" chemicals&mdash;are of no concern whatsoever. In off-the-shelf chemicals the concentrations of different isotopologues are determined by the terrestrial ''natural abundances'' of the isotopes. Take the element [[chlorine]] as an example. It has two stable isotopes:&nbsp; <sup>35</sup>Cl (with a mass of 34.96885271 u) and <sup>37</sup>Cl (with a mass of  36.96590260 u). Of all the chlorine atoms occurring on earth  75.78 % is of the lighter kind, while  24.22 % is the heavier isotope. The average mass of the Cl atom is 
::: (34.969&times;75.78 + 36.966&times;24.22)/100 = 35.453 u.

: The atomic mass averaged over isotopic abundances is called the  '''standard atomic weight'''. (For historical reasons the term "weight" is  still used here.) In most of practical chemistry the standard weight is used as "the" mass of an element. By using averaged masses the chemist accounts for the fact that different isotopes occur in nature. For instance, the HCl molecule has standard atomic weight 1.00794 + 35.453 = 36.461, which is the value used in almost all chemical calculations.

==Note on nomenclature==
The concept of "relative atomic mass" is in principle a simple one, yet there is some confusion about its definition.  [http://physics.nist.gov/PhysRefData/Compositions/notes.html NIST] clearly and unambiguously defines the ''relative mass'' of an ''isotope'':
<blockquote>
'''Relative Atomic Mass (of the isotope):''' ''A''<sub>r</sub>(X), where X is an isotope
</blockquote>
This usage is  followed by Mohr and Taylor,<ref>P. J. Mohr and B. N. Taylor, ''CODATA recommended values of the fundamental physical constants: 2002'', Reviews of Modern Physics, vol. '''77''', p. 1  (2005)</ref> who state that (the [[atomic mass constant]] ''m''<sub>u</sub> is a twelfth of the mass of <sup>12</sup>C): 
<blockquote>
The relative atomic mass ''A''<sub>r</sub>(X) of an elementary particle, atom, or more generally an entity X, is defined by ''A''<sub>r</sub>(X) = ''m''(X) / ''m''<sub>u</sub>, where ''m''(X) is the mass of X. Thus ''A''<sub>r</sub>(X) is the numerical value of ''m''(X) when ''m''(X) is expressed
in u, and evidently ''A''<sub>r</sub>(<sup>12</sup>C)=12.
</blockquote>
On the other hand, the official [[IUPAC]]  publication, [http://www.iupac.org/goldbook/R05258.pdf IUPAC Goldbook], defines:
<blockquote>
'''relative atomic mass (atomic weight)''', ''A''<sub>r</sub> <br/>
The ratio of the average mass of the atom to the unified atomic mass unit 
</blockquote>
Although it is not stated  explicitly in the Goldbook what is meant by "average mass", it is likely and plausible that the averaging is over different isotopes weighted by terrestrial isotopic abundance. Hence, according to IUPAC's definition, the ''relative atomic mass'' is the dimensionless version of the ''standard atomic weight'' defined above. 

IUPAC also defines ''standard atomic weight'', but  adds ''recommended'' to its definition, that is, IUPAC defines standard atomic weight as  ''recommended relative atomic mass'', which suggests that the recommended value may change in the future when more accurate data become available.

The confusion is created by too many international committees addressing the same, basically very simple, problem of definition.<ref> [http://www.iupac.org/publications/pac/1992/pdf/6410x1535.pdf Article about Atomic Weights] </ref>

==Standard atomic weights of the elements==
The following table<ref>[http://physics.nist.gov/PhysRefData/Compositions/index.html Physical Reference Data]. The numbers in this table are taken from the web site of [[NIST]] on December 2, 2007.</ref> lists the standard atomic weights of the chemical elements (= for each element, the average atomic mass of all isotopes each weighted by their terrestrial abundance). The uncertainties in the last given decimal are in parentheses.  Parenthesis around the entire number indicate the mass number of the most stable isotope. CS stands for chemical symbol. ''Z'' is the [[atomic number]]. See [[Chemical elements|here]] for a list of the full names of the elements.

<div align="center">
<table width="80%">
<!-- <caption><h3>Standard Atomic Weights of the Elements</h3> </caption> -->
<tr><th colspan="12"><hr></tr>
<tr>
    <th align="left">Z<th align="left">CS <th align="left">Mass
<th><th align="left">Z<th align="left">CS <th align="left">Mass
<th><th align="left">Z<th align="left">CS <th align="left">Mass
</tr>
<tr><th colspan="12"><hr> </tr>
<tr><td align="left ">  1  <td>   [[Hydrogen|H]]    <td width="5%" >  [[Hydrogen/Atomic mass|{{:Hydrogen/Atomic mass}}]]    <td width="15%"> <td align="left "> 38  <td>   [[Strontium|Sr]]  <td width="5%" >  [[Strontium/Atomic mass|{{:Strontium/Atomic mass}}]]       <td width="15%">  <td align="left "> 75  <td>   [[Rhenium|Re]]   <td width="5%" >   [[Rhenium/Atomic mass|{{:Rhenium/Atomic mass}}]]
<tr><td align="left ">  2  <td>   [[Helium|He]]   <td width="5%" >  [[Helium/Atomic mass|{{:Helium/Atomic mass}}]]   <td width="15%"> <td align="left "> 39  <td>   [[Yttrium|Y]]   <td width="5%" >  [[Yttrium/Atomic mass|{{:Yttrium/Atomic mass}}]]    <td width="15%">  <td align="left "> 76  <td>   [[Osmium|Os]]   <td width="5%" >   [[Osmium/Atomic mass|{{:Osmium/Atomic mass}}]]
<tr><td align="left ">  3  <td>   [[Lithium|Li]]   <td width="5%" >  [[Lithium/Atomic mass|{{:Lithium/Atomic mass}}]]      <td width="15%"> <td align="left "> 40  <td>   [[Zirconium|Zr]]  <td width="5%" >  [[Zirconium/Atomic mass|{{:Zirconium/Atomic mass}}]]      <td width="15%">  <td align="left "> 77  <td>   [[Iridium|Ir]]   <td width="5%" >   [[Iridium/Atomic mass|{{:Iridium/Atomic mass}}]]
<tr><td align="left ">  4  <td>   [[Beryllium|Be]]   <td width="5%" >  [[Beryllium/Atomic mass|{{:Beryllium/Atomic mass}}]]   <td width="15%"> <td align="left "> 41  <td>   [[Niobium|Nb]]  <td width="5%" >  [[Niobium/Atomic mass|{{:Niobium/Atomic mass}}]]    <td width="15%">  <td align="left "> 78  <td>   [[Platinum|Pt]]   <td width="5%" >   [[Platinum/Atomic mass|{{:Platinum/Atomic mass}}]]
<tr><td align="left ">  5  <td>   [[Boron|B]]    <td width="5%" >  {{Atomic mass|Boron}}     <td width="15%"> <td align="left "> 42  <td>   [[Molybdenum|Mo]]  <td width="5%" >  {{Atomic mass|Molybdenum}}       <td width="15%">  <td align="left "> 79  <td>   [[Gold|Au]]   <td width="5%" >   {{Atomic mass|Gold}}
<tr><td align="left ">  6  <td>   [[Carbon|C]]    <td width="5%" >  {{Atomic mass|Carbon}}    <td width="15%"> <td align="left "> 43  <td>   [[Technetium|Tc]]  <td width="5%" >  {{Atomic mass|Technetium}}           <td width="15%">  <td align="left "> 80  <td>   [[Mercury (element)|Hg]]   <td width="5%" >   {{Atomic mass|Mercury (element)}}
<tr><td align="left ">  7  <td>   [[Nitrogen|N]]    <td width="5%" >  {{Atomic mass|Nitrogen}}    <td width="15%"> <td align="left "> 44  <td>   [[Ruthenium|Ru]]  <td width="5%" >  {{Atomic mass|Ruthenium}}      <td width="15%">  <td align="left "> 81  <td>   [[Thallium|Tl]]   <td width="5%" >   {{Atomic mass|Thallium}}
<tr><td align="left ">  8  <td>   [[Oxygen|O]]    <td width="5%" >  {{Atomic mass|Oxygen}}    <td width="15%"> <td align="left "> 45  <td>   [[Rhodium|Rh]]  <td width="5%" >  {{Atomic mass|Rhodium}}   <td width="15%">  <td align="left "> 82  <td>   [[Lead|Pb]]   <td width="5%" >   {{Atomic mass|Lead}}
<tr><td align="left ">  9  <td>   [[Fluorine|F]]    <td width="5%" >  {{Atomic mass|Fluorine}} <td width="15%"> <td align="left "> 46  <td>   [[Palladium|Pd]]  <td width="5%" >  {{Atomic mass|Palladium}}      <td width="15%">  <td align="left "> 83  <td>   [[Bismuth|Bi]]   <td width="5%" >   {{Atomic mass|Bismuth}}
<tr><td align="left "> 10  <td>   [[Neon|Ne]]   <td width="5%" >  {{Atomic mass|Neon}}    <td width="15%"> <td align="left "> 47  <td>   [[Silver|Ag]]  <td width="5%" >  {{Atomic mass|Silver}}    <td width="15%">  <td align="left "> 84  <td>   [[Polonium|Po]]   <td width="5%" >   {{Atomic mass|Polonium}}
<tr><td align="left "> 11  <td>   [[Sodium|Na]]   <td width="5%" >  {{Atomic mass|Sodium}}  <td width="15%"> <td align="left "> 48  <td>   [[Cadmium|Cd]]  <td width="5%" >  {{Atomic mass|Cadmium}}     <td width="15%">  <td align="left "> 85  <td>   [[Astatine|At]]   <td width="5%" >   {{Atomic mass|Astatine}}
<tr><td align="left "> 12  <td>   [[Magnesium|Mg]]   <td width="5%" >  {{Atomic mass|Magnesium}}    <td width="15%"> <td align="left "> 49  <td>   [[Indium|In]]  <td width="5%" >  {{Atomic mass|Indium}}     <td width="15%">  <td align="left "> 86  <td>   [[Radon|Rn]]   <td width="5%" >   {{Atomic mass|Radium}}
<tr><td align="left "> 13  <td>   [[Aluminum|Al]]   <td width="5%" >  {{Atomic mass|Aluminum}}  <td width="15%"> <td align="left "> 50  <td>   [[Tin|Sn]]  <td width="5%" >  {{Atomic mass|Tin}}     <td width="15%">  <td align="left "> 87  <td>   [[Francium|Fr]]   <td width="5%" >   {{Atomic mass|Francium}}
<tr><td align="left "> 14  <td>   [[Silicon|Si]]   <td width="5%" >  {{Atomic mass|Silicon}}    <td width="15%"> <td align="left "> 51  <td>   [[Antimony|Sb]]  <td width="5%" >  {{Atomic mass|Antimony}}     <td width="15%">  <td align="left "> 88  <td>   [[Radium|Ra]]   <td width="5%" >   {{Atomic mass|Radium}}
<tr><td align="left "> 15  <td>   [[Phosphorus|P]]    <td width="5%" >  {{Atomic mass|Phosphorus}}  <td width="15%"> <td align="left "> 52  <td>   [[Tellurium|Te]]  <td width="5%" >  {{Atomic mass|Tellurium}}      <td width="15%">  <td align="left "> 89  <td>   [[Actinium|Ac]]   <td width="5%" >   {{Atomic mass|Actinium}}
<tr><td align="left "> 16  <td>   [[Sulfur|S]]    <td width="5%" >  {{Atomic mass|Sulfur}}     <td width="15%"> <td align="left "> 53  <td>   [[Iodine|I]]   <td width="5%" >  {{Atomic mass|Iodine}}   <td width="15%">  <td align="left "> 90  <td>   [[Thorium|Th]]   <td width="5%" >   {{Atomic mass|Thorium}}
<tr><td align="left "> 17  <td>   [[Chlorine|Cl]]   <td width="5%" >  {{Atomic mass|Chlorine}}     <td width="15%"> <td align="left "> 54  <td>   [[Xenon|Xe]]  <td width="5%" >  {{Atomic mass|Xenon}}     <td width="15%">  <td align="left "> 91  <td>   [[Protactinium|Pa]]   <td width="5%" >   {{Atomic mass|Protactinium}}
<tr><td align="left "> 18  <td>   [[Argon|Ar]]   <td width="5%" >  {{Atomic mass|Argon}}     <td width="15%"> <td align="left "> 55  <td>   [[Caesium|Cs]]  <td width="5%" >  {{Atomic mass|Caesium}}   <td width="15%">  <td align="left "> 92  <td>   [[Uranium|U]]    <td width="5%" >   {{Atomic mass|Uranium}}
<tr><td align="left "> 19  <td>   [[Potassium|K]]    <td width="5%" >  {{Atomic mass|Potassium}}    <td width="15%"> <td align="left "> 56  <td>   [[Barium|Ba]]  <td width="5%" >  {{Atomic mass|Barium}}     <td width="15%">  <td align="left "> 93  <td>   [[Neptunium|Np]]   <td width="5%" >   {{Atomic mass|Neptunium}}
<tr><td align="left "> 20  <td>   [[Calcium|Ca]]   <td width="5%" >  {{Atomic mass|Calcium}}     <td width="15%"> <td align="left "> 57  <td>   [[Lanthanum|La]]  <td width="5%" >  {{Atomic mass|Lanthanum}}    <td width="15%">  <td align="left "> 94  <td>   [[Plutonium|Pu]]   <td width="5%" >   {{Atomic mass|Plutonium}}
<tr><td align="left "> 21  <td>   [[Scandium|Sc]]   <td width="5%" >  {{Atomic mass|Scandium}}  <td width="15%"> <td align="left "> 58  <td>   [[Cerium|Ce]]  <td width="5%" >  {{Atomic mass|Cerium}}     <td width="15%">  <td align="left "> 95  <td>   [[Americium|Am]]   <td width="5%" >   {{Atomic mass|Americium}}
<tr><td align="left "> 22  <td>   [[Titanium|Ti]]   <td width="5%" >  {{Atomic mass|Titanium}}     <td width="15%"> <td align="left "> 59  <td>   [[Praseodymium|Pr]]  <td width="5%" >  {{Atomic mass|Praseodymium}}   <td width="15%">  <td align="left "> 96  <td>   [[Curium|Cm]]   <td width="5%" >   {{Atomic mass|Curium}}
<tr><td align="left "> 23  <td>   [[Vanadium|V]]    <td width="5%" >  {{Atomic mass|Vanadium}}    <td width="15%"> <td align="left "> 60  <td>   [[Neodymium|Nd]]  <td width="5%" >  {{Atomic mass|Neodymium}}      <td width="15%">  <td align="left "> 97  <td>   [[Berkelium|Bk]]   <td width="5%" >   {{Atomic mass|Berkelium}}
<tr><td align="left "> 24  <td>   [[Chromium|Cr]]   <td width="5%" >  {{Atomic mass|Chromium}}    <td width="15%"> <td align="left "> 61  <td>   [[Promethium|Pm]]  <td width="5%" >  {{Atomic mass|Promethium}}          <td width="15%">  <td align="left "> 98  <td>   [[Californium|Cf]]   <td width="5%" >   {{Atomic mass|Californium}}
<tr><td align="left "> 25  <td>   [[Manganese|Mn]]   <td width="5%" >  {{Atomic mass|Manganese}}  <td width="15%"> <td align="left "> 62  <td>   [[Samarium|Sm]]  <td width="5%" >  {{Atomic mass|Samarium}}      <td width="15%">  <td align="left "> 99  <td>   [[Einsteinium|Es]]   <td width="5%" >   {{Atomic mass|Einsteinium}}
<tr><td align="left "> 26  <td>   [[Iron|Fe]]   <td width="5%" >  {{Atomic mass|Iron}}     <td width="15%"> <td align="left "> 63  <td>   [[Europium|Eu]]  <td width="5%" >  {{Atomic mass|Europium}}     <td width="15%">  <td align="left ">100  <td>   [[Fermium|Fm]]   <td width="5%" >   {{Atomic mass|Fermium}}
<tr><td align="left "> 27  <td>   [[Cobalt|Co]]   <td width="5%" >  {{Atomic mass|Cobalt}}  <td width="15%"> <td align="left "> 64  <td>   [[Gadolinium|Gd]]  <td width="5%" >  {{Atomic mass|Gadolinium}}      <td width="15%">  <td align="left ">101  <td>   [[Mendelevium|Md]]   <td width="5%" >   {{Atomic mass|Mendelevium}}
<tr><td align="left "> 28  <td>   [[Nickel|Ni]]   <td width="5%" >  {{Atomic mass|Nickel}}    <td width="15%"> <td align="left "> 65  <td>   [[Terbium|Tb]]  <td width="5%" >  {{Atomic mass|Terbium}}   <td width="15%">  <td align="left ">102  <td>   [[Nobelium|No]]   <td width="5%" >   {{Atomic mass|Nobelium}}
<tr><td align="left "> 29  <td>   [[Copper|Cu]]   <td width="5%" >  {{Atomic mass|Copper}}     <td width="15%"> <td align="left "> 66  <td>   [[Dysprosium|Dy]]  <td width="5%" >  {{Atomic mass|Dysprosium}}     <td width="15%">  <td align="left ">103  <td>   [[Lawrencium|Lr]]   <td width="5%" >   {{Atomic mass|Lawrencium}}
<tr><td align="left "> 30  <td>   [[Zinc|Zn]]   <td width="5%" >  {{Atomic mass|Zinc}}     <td width="15%"> <td align="left "> 67  <td>   [[Holmium|Ho]]  <td width="5%" >  {{Atomic mass|Holmium}}   <td width="15%">  <td align="left ">104  <td>   [[Rutherfordium|Rf]]   <td width="5%" >   {{Atomic mass|Rutherfordium}}
<tr><td align="left "> 31  <td>   [[Gallium|Ga]]   <td width="5%" >  {{Atomic mass|Gallium}}     <td width="15%"> <td align="left "> 68  <td>   [[Erbium|Er]]  <td width="5%" >  {{Atomic mass|Erbium}}     <td width="15%">  <td align="left ">105  <td>   [[Dubnium|Db]]   <td width="5%" >   {{Atomic mass|Dubnium}}
<tr><td align="left "> 32  <td>   [[Germanium|Ge]]   <td width="5%" >  {{Atomic mass|Germanium}}      <td width="15%"> <td align="left "> 69  <td>   [[Thulium|Tm]]  <td width="5%" >  {{Atomic mass|Thulium}}   <td width="15%">  <td align="left ">106  <td>   [[Seaborgium|Sg]]   <td width="5%" >   {{Atomic mass|Seaborgium}}
<tr><td align="left "> 33  <td>   [[Arsenic|As]]   <td width="5%" >  {{Atomic mass|Arsenic}}   <td width="15%"> <td align="left "> 70  <td>   [[Ytterbium|Yb]]  <td width="5%" >  {{Atomic mass|Ytterbium}}      <td width="15%">  <td align="left ">107  <td>   [[Bohrium|Bh]]   <td width="5%" >   {{Atomic mass|Bohrium}}
<tr><td align="left "> 34  <td>   [[Selenium|Se]]   <td width="5%" >  {{Atomic mass|Selenium}}      <td width="15%"> <td align="left "> 71  <td>   [[Lutetium|Lu]]  <td width="5%" >  {{Atomic mass|Lutetium}}     <td width="15%">  <td align="left ">108  <td>   [[Hassium|Hs]]   <td width="5%" >   {{Atomic mass|Hassium}}
<tr><td align="left "> 35  <td>   [[Bromine|Br]]   <td width="5%" >  {{Atomic mass|Bromine}}     <td width="15%"> <td align="left "> 72  <td>   [[Hafnium|Hf]]  <td width="5%" >  {{Atomic mass|Hafnium}}      <td width="15%">  <td align="left ">109  <td>   [[Meitnerium|Mt]]   <td width="5%" >   {{Atomic mass|Meitnerium}}
<tr><td align="left "> 36  <td>   [[Krypton|Kr]]   <td width="5%" >  {{Atomic mass|Krypton}}     <td width="15%"> <td align="left "> 73  <td>   [[Tantalum|Ta]]  <td width="5%" >  {{Atomic mass|Tantalum}}    <td width="15%">  <td align="left ">110  <td>   [[Darmstadtium|Ds]]   <td width="5%" >   {{Atomic mass|Darmstadtium}}
<tr><td align="left "> 37  <td>   [[Rubidium|Rb]]   <td width="5%" >  {{Atomic mass|Rubidium}}    <td width="15%"> <td align="left "> 74  <td>   [[Tungsten|W]]   <td width="5%" >  {{Atomic mass|Tungsten}}      <td width="15%">  <td align="left ">111  <td>   [[Roentgenium|Rg]]   <td width="5%" >   {{Atomic mass|Roentgenium}}
<tr><td colspan="12" ><hr> </tr>
</table>
</div>

==Notes==
{{reflist|2}}