Atomic mass: Difference between revisions
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In [[chemistry]], the '''atomic mass''' (formerly '''atomic weight''') is the [[mass]] of an atom expressed in [[unified atomic mass unit]]s (u). The atomic mass is equal in value to relative atomic mass, ''A''<sub>r</sub>(X), where X is an [[isotope]]. While atomic mass has the dimension u, relative atomic mass—the proportion of an atomic mass to one twelfth of the mass of <sup>12</sup>C—is dimensionless. | In [[chemistry]] and [[physics]], the '''atomic mass''' (formerly '''atomic weight''') is the [[mass]] of an atom expressed in [[unified atomic mass unit]]s (u). The atomic mass is equal in value to relative atomic mass, ''A''<sub>r</sub>(X), where X is an [[isotope]]. While atomic mass has the dimension u, relative atomic mass—the proportion of an atomic mass to one twelfth of the mass of <sup>12</sup>C—is dimensionless. | ||
Different isotopes of an atom have different numbers of neutrons in the atomic nucleus, while, by definition, an atomic nucleus has a fixed number of protons. Different isotopes of the same atom have different masses, due to the differing number of neutrons. For instance, [[carbon]] (six protons) has two stable isotopes and one radioactive—but long-lived—isotope. Their 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 atomic mass of <sup>12</sup>C is by definition the integral number 12. | Different isotopes of an atom have different numbers of neutrons in the atomic nucleus, while, by definition, an atomic nucleus has a fixed number of protons. Different isotopes of the same atom have different masses, due to the differing number of neutrons. For instance, [[carbon]] (six protons) has two stable isotopes and one radioactive—but long-lived—isotope. Their 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 <sup>12</sup>C is by definition the integral number 12. By the same definition the atomic mass is 12 u. | ||
In [[high resolution spectroscopy]] masses of different isotopes are observed in the spectra, and in this field computations are usually done for [[molecule]]s consisting of well defined isotopes. In [[chemistry]] this is different. Chemicals used in the laboratory are in general isotopic mixtures: their molecules consist of different isotopes of one and the same element. The proportion of different isotopes in the molecule is determined by the ''natural abundance'' of the isotope. | In [[high resolution spectroscopy]] masses of different isotopes are observed in the spectra, and in this field computations are usually done for [[molecule]]s consisting of well defined isotopes. In [[chemistry]] this is different. Chemicals used in the laboratory are in general isotopic mixtures: their molecules consist of different isotopes of one and the same element. The proportion of different isotopes in the molecule is determined by the ''natural abundance'' of the isotope. | ||
Take [[chlorine]] as an example. This element has two stable isotopes: <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. | Take [[chlorine]] as an example. This element has two stable isotopes: <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 thus (34. | The average mass of the Cl atom is thus (34.969×75.78 + 36.966×24.22)/100 = 35.453 u. | ||
The atomic mass averaged over isotopic abundances is called the '''standard atomic weight'''. (For historical reasons the term "weight" is used here.) | The atomic mass averaged over isotopic abundances is called the '''standard atomic weight'''. (For historical reasons the term "weight" is used here.) | ||
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The ratio of the average mass of the atom to the unified atomic mass unit | The ratio of the average mass of the atom to the unified atomic mass unit | ||
</blockquote> | </blockquote> | ||
Although it is not explicitly stated here what the average mass is, it is plausible that the averaging | Although it is not explicitly stated here what the average mass is, it is plausible that the averaging is over different isotopes weighted by terrestrial isotopic abundance. Hence, acccording to IUPAC's definition, the ''relative atomic mass'' is nearly synonymous with the ''standard atomic weight'' defined above. In IUPAC's definition, a standard atomic weight is a ''recommended'' relative atomic mass, which means that IUPAC's standard atomic weight will change over time (because recommendations change regularly), but that IUPAC's relative atomic mass is invariant in time. | ||
Ref. <ref> [http://www.iupac.org/publications/pac/1992/pdf/6410x1535.pdf Article about Atomic Weights] </ref> makes it clear that this—messy and unnecessary—confusion is created by too many international comittees addressing this, basically very simple, problem. | Ref. <ref> [http://www.iupac.org/publications/pac/1992/pdf/6410x1535.pdf Article about Atomic Weights] </ref> makes it clear that this—messy and unnecessary—confusion is created by too many international comittees addressing this, basically very simple, problem. | ||
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[[Category: CZ Live]] | [[Category: CZ Live]] | ||
[[Category: Chemistry Workgroup]] | [[Category: Chemistry Workgroup]] | ||
[[Category: Physics Workgroup]] | |||
Revision as of 04:16, 3 December 2007
In chemistry and physics, the atomic mass (formerly atomic weight) is the mass of an atom expressed in unified atomic mass units (u). The atomic mass is equal in value to relative atomic mass, Ar(X), where X is an isotope. While atomic mass has the dimension u, relative atomic mass—the proportion of an atomic mass to one twelfth of the mass of 12C—is dimensionless.
Different isotopes of an atom have different numbers of neutrons in the atomic nucleus, while, by definition, an atomic nucleus has a fixed number of protons. Different isotopes of the same atom have different masses, due to the differing number of neutrons. For instance, carbon (six protons) has two stable isotopes and one radioactive—but long-lived—isotope. Their respective atomic masses are, 12C: 12 u (six neutrons), 13C: 13.0033548378 u (seven neutrons), and 14C: 14.003241988 u (eight neutrons). The relative atomic mass of 12C is by definition the integral number 12. By the same definition the atomic mass is 12 u.
In high resolution spectroscopy masses of different isotopes are observed in the spectra, and in this field computations are usually done for molecules consisting of well defined isotopes. In chemistry this is different. Chemicals used in the laboratory are in general isotopic mixtures: their molecules consist of different isotopes of one and the same element. The proportion of different isotopes in the molecule is determined by the natural abundance of the isotope. Take chlorine as an example. This element has two stable isotopes: 35Cl (with a mass of 34.96885271 u) and 37Cl (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 thus (34.969×75.78 + 36.966×24.22)/100 = 35.453 u.
The atomic mass averaged over isotopic abundances is called the standard atomic weight. (For historical reasons the term "weight" is used here.)
Note on nomenclature
Although "relative atomic mass" is in principle a simple concept, unfortunately there is confusion about its definition. We followed the lead of NIST, see the NIST web site, where clearly and unambiguously the relative mass is defined of an isotope. The site states:
Relative Atomic Mass (of the isotope): Ar(X), where X is an isotope
However, the official IUPAC publication, IUPAC Goldbook, defines:
relative atomic mass (atomic weight), Ar
The ratio of the average mass of the atom to the unified atomic mass unit
Although it is not explicitly stated here what the average mass is, it is plausible that the averaging is over different isotopes weighted by terrestrial isotopic abundance. Hence, acccording to IUPAC's definition, the relative atomic mass is nearly synonymous with the standard atomic weight defined above. In IUPAC's definition, a standard atomic weight is a recommended relative atomic mass, which means that IUPAC's standard atomic weight will change over time (because recommendations change regularly), but that IUPAC's relative atomic mass is invariant in time.
Ref. [1] makes it clear that this—messy and unnecessary—confusion is created by too many international comittees addressing this, basically very simple, problem.
Standard Atomic Weights of the Elements
A table [2] is given for the standard atomic weights. Brackets [ ] indicate the mass number of the most stable isotope. CS stands for chemical symbol. Z is the atomic number. See this article for a list of the full names of the elements.
| Z | CS | Mass | Z | CS | Mass | Z | CS | Mass | |||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | H | 1.00794(7) | 38 | Sr | 87.62(1) | 75 | Re | 186.207(1) | |||
| 2 | He | 4.002602(2) | 39 | Y | 88.90585(2) | 76 | Os | 190.23(3) | |||
| 3 | Li | 6.941(2) | 40 | Zr | 91.224(2) | 77 | Ir | 192.217(3) | |||
| 4 | Be | 9.012182(3) | 41 | Nb | 92.90638(2) | 78 | Pt | 195.078(2) | |||
| 5 | B | 10.811(7) | 42 | Mo | 95.94(2) | 79 | Au | 196.96655(2) | |||
| 6 | C | 12.0107(8) | 43 | Tc | [98] | 80 | Hg | 200.59(2) | |||
| 7 | N | 14.0067(2) | 44 | Ru | 101.07(2) | 81 | Tl | 204.3833(2) | |||
| 8 | O | 15.9994(3) | 45 | Rh | 102.90550(2) | 82 | Pb | 207.2(1) | |||
| 9 | F | 18.9984032(5) | 46 | Pd | 106.42(1) | 83 | Bi | 208.98038(2) | |||
| 10 | Ne | 20.1797(6) | 47 | Ag | 107.8682(2) | 84 | Po | [209] | |||
| 11 | Na | 22.989770(2) | 48 | Cd | 112.411(8) | 85 | At | [210] | |||
| 12 | Mg | 24.3050(6) | 49 | In | 114.818(3) | 86 | Rn | [222] | |||
| 13 | Al | 26.981538(2) | 50 | Sn | 118.710(7) | 87 | Fr | [223] | |||
| 14 | Si | 28.0855(3) | 51 | Sb | 121.760(1) | 88 | Ra | [226] | |||
| 15 | P | 30.973761(2) | 52 | Te | 127.60(3) | 89 | Ac | [227] | |||
| 16 | S | 32.065(5) | 53 | I | 126.90447(3) | 90 | Th | 232.0381(1) | |||
| 17 | Cl | 35.453(2) | 54 | Xe | 131.293(6) | 91 | Pa | 231.03588(2) | |||
| 18 | Ar | 39.948(1) | 55 | Cs | 132.90545(2) | 92 | U | 238.02891(3) | |||
| 19 | K | 39.0983(1) | 56 | Ba | 137.327(7) | 93 | Np | [237] | |||
| 20 | Ca | 40.078(4) | 57 | La | 138.9055(2) | 94 | Pu | [244] | |||
| 21 | Sc | 44.955910(8) | 58 | Ce | 140.116(1) | 95 | Am | [243] | |||
| 22 | Ti | 47.867(1) | 59 | Pr | 140.90765(2) | 96 | Cm | [247] | |||
| 23 | V | 50.9415(1) | 60 | Nd | 144.24(3) | 97 | Bk | [247] | |||
| 24 | Cr | 51.9961(6) | 61 | Pm | [145] | 98 | Cf | [251] | |||
| 25 | Mn | 54.938049(9) | 62 | Sm | 150.36(3) | 99 | Es | [252] | |||
| 26 | Fe | 55.845(2) | 63 | Eu | 151.964(1) | 100 | Fm | [257] | |||
| 27 | Co | 58.933200(9) | 64 | Gd | 157.25(3) | 101 | Md | [258] | |||
| 28 | Ni | 58.6934(2) | 65 | Tb | 158.92534(2) | 102 | No | [259] | |||
| 29 | Cu | 63.546(3) | 66 | Dy | 162.500(1) | 103 | Lr | [262] | |||
| 30 | Zn | 65.409(4) | 67 | Ho | 164.93032(2) | 104 | Rf | [261] | |||
| 31 | Ga | 69.723(1) | 68 | Er | 167.259(3) | 105 | Db | [262] | |||
| 32 | Ge | 72.64(1) | 69 | Tm | 168.93421(2) | 106 | Sg | [266] | |||
| 33 | As | 74.92160(2) | 70 | Yb | 173.04(3) | 107 | Bh | [264] | |||
| 34 | Se | 78.96(3) | 71 | Lu | 174.967(1) | 108 | Hs | [277] | |||
| 35 | Br | 79.904(1) | 72 | Hf | 178.49(2) | 109 | Mt | [268] | |||
| 36 | Kr | 83.798(2) | 73 | Ta | 180.9479(1) | 110 | Ds | [281] | |||
| 37 | Rb | 85.4678(3) | 74 | W | 183.84(1) | 111 | Rg | [272] | |||
Notes
- ↑ Article about Atomic Weights
- ↑ The numbers in this table are taken from the web site of NIST on December 2 2007. Physical Reference Data.