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]], 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 | 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. | ||
has two stable isotopes and one radioactive—but long-lived—isotope. Their respective atomic masses are, <sup>12</sup>C: 12 u, <sup>13</sup>C: 13.0033548378 u, and <sup>14</sup>C: 14.003241988 u. The atomic mass of <sup>12</sup>C is by definition the integral number 12. | |||
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. | ||
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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.) | ||
==Note on nomenclature== | |||
Although relative atomic mass is a simple concept, there is confusion about its nomenclature. We followed the lead of [[NIST]], see the [http://physics.nist.gov/PhysRefData/Compositions/notes.html NIST web site], where clearly and unambiguously the ''relative mass'' is defined of an ''isotope''. The site states: | |||
<blockquote> | |||
'''Relative Atomic Mass (of the isotope):''' ''A''<sub>r</sub>(X), where X is an isotope | |||
</blockquote> | |||
However, 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 explicitly stated here what the average mass is, it is very probable that the averaging is over different isotopes weighted by terrestrial abundance. Hence, acccording to IUPAC's definition ''relative atomic mass'' is synonomous to ''standard atomic weight'' defined above. From Ref. <ref> [http://www.iupac.org/publications/pac/1992/pdf/6410x1535.pdf Article about Atomic Weights] </ref> it is clear that this confusion is created by too many international comittees addressing this, basically very simple, problem. | |||
==Standard Atomic Weights of the Elements== | |||
A table <ref> The numbers in this table are taken from the web site of [[NIST]] on December 2 2007. | |||
[http://physics.nist.gov/PhysRefData/Compositions/index.html Physical Reference Data].</ref> 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]]. | |||
---- | ---- | ||
<div align="center"> | <div align="center"> | ||
<table width="80%"> | <table width="80%"> | ||
<caption><h3>Standard Atomic Weights of the Elements</h3> </caption> | <!-- <caption><h3>Standard Atomic Weights of the Elements</h3> </caption> --> | ||
<tr><th colspan="12"><hr></tr> | <tr><th colspan="12"><hr></tr> | ||
<tr> | <tr> | ||
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==Note== | ==Note== | ||
<references /> | <references /> | ||
[[Category: CZ Live]] | [[Category: CZ Live]] | ||
[[Category: Chemistry Workgroup]] | [[Category: Chemistry Workgroup]] | ||
Revision as of 02:48, 3 December 2007
In chemistry, 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 atomic mass of 12C is by definition the integral number 12.
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 (34.96885271×75.78 + 36.96590260×24.22)/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 a simple concept, there is confusion about its nomenclature. 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 very probable that the averaging is over different isotopes weighted by terrestrial abundance. Hence, acccording to IUPAC's definition relative atomic mass is synonomous to standard atomic weight defined above. From Ref. [1] it is clear that this 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.
| 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] | |||
Note
- ↑ Article about Atomic Weights
- ↑ The numbers in this table are taken from the web site of NIST on December 2 2007. Physical Reference Data.