Atomic mass: Difference between revisions
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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.) | ||
Below a table | Below 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].</ref> | [http://physics.nist.gov/PhysRefData/Compositions/index.html].</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]]. | ||
Revision as of 12:14, 2 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 are characterized by a different numbers of neutrons in the atomic nucleus. Hence, different isotopes of the same atom have different masses. For instance, carbon has two stable isotopes and one radioactive—but long-lived—isotope. Their respective atomic masses are, 12C: 12 u, 13C: 13.0033548378 u, and 14C: 14.003241988 u. 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.)
Below a table [1] 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] | |||