Atomic mass
Atomic mass (formerly atomic weight) is the mass of an atom expressed in unified atomic mass units (u). That is, an atomic mass is expressed relative to a twelfth of the mass of the 12C atom. Therefore, atomic masses are referred to sometimes as relative atomic masses, symbol Ar(X), where X is an isotope.
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 masses are, 12C: 12 u, 13C: 13.0033548378 u, and 14C: 14.003241988 u. The 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. This average mass is the standard atomic mass.
Below a table is given for the standard atomic masses. Brackets [ ] indicate the mass number of the most stable isotope. CS stands for chemical symbol. Z is the atomic number. The numbers in this table are from the web site of NIST: physics reference data.
| 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] | |||