# 26.1: Appendix A. Standard Atomic Weights 1999†

[Scaled to Ar(12C) = 12, where 12C is a neutral atom in its nuclear and electronic ground state]

$\begin{array}{l l l l} \text{Atomic Number} & \text{Name} & \text{Symbol} & \text{Atomic Weight} \\ \hline 1 & \text{Hydrogen} & \text{H} & 1.00794 \\ 2 & \text{Helium} & \text{He} & 4.002602 \\ 3 & \text{Lithium} & \text{Li} & [6.941(2)] \\ 4 & \text{Beryllium} & \text{Be} & 9.012182 \\ 5 & \text{Boron} & \text{B} & 10.811 \\ 6 & \text{Carbon} & \text{C} & 12.0107 \\ 7 & \text{Nitrogen} & \text{N} & 14.0067 \\ 8 & \text{Oxygen} & \text{O} & 15.9994 \\ 9 & \text{Fluorine} & \text{F} & 18.9984032 \\ 10 & \text{Neon} & \text{Ne} & 20.1797 \\ 11 & \text{Sodium} & \text{Na} & 22.989770 \\ 12 & \text{Magnesium} & \text{Mg} & 24.3050 \\ 13 & \text{Aluminum} & \text{Al} & 26.981538 \\ 14 & \text{Silicon} & \text{Si} & 28.0855 \\ 15 & \text{Phosphorus} & \text{P} & 30.973761 \\ 16 & \text{Sulfur} & \text{S} & 32.065 \\ 17 & \text{Chlorine} & \text{Cl} & 35.453 \\ 18 & \text{Argon} & \text{Ar} & 39.948 \\ 19 & \text{Potassium} & \text{K} & 39.0983 \\ 20 & \text{Calcium} & \text{Ca} & 40.078 \\ 21 & \text{Scandium} & \text{Sc} & 44.955910 \\ 22 & \text{Titanium} & \text{Ti} & 47.867 \\ 23 & \text{Vanadium} & \text{V} & 50.9415 \\ 24 & \text{Chromium} & \text{Cr} & 51.9961 \\ 25 & \text{Manganese} & \text{Mn} & 54.938049 \\ 26 & \text{Iron} & \text{Fe} & 55.845 \\ 27 & \text{Cobalt} & \text{Co} & 58.933200 \\ 28 & \text{Nickel} & \text{Ni} & 58.6934 \\ 29 & \text{Copper} & \text{Cu} & 63.546 \\ 30 & \text{Zinc} & \text{Zn} & 65.39 \\ 31 & \text{Gallium} & \text{Ga} & 69.723 \\ 32 & \text{Germanium} & \text{Ge} & 72.64 \\ 33 & \text{Arsenic} & \text{As} & 74.92160 \\ 34 & \text{Selenium} & \text{Se} & 78.96 \\ 35 & \text{Bromine} & \text{Br} & 79.904 \\ 36 & \text{Krypton} & \text{Kr} & 83.80 \\ 37 & \text{Rubidium} & \text{Rb} & 85.4678 \\ 38 & \text{Strontium} & \text{Sr} & 87.62 \\ 39 & \text{Yttrium} & \text{Y} & 88.90585 \\ 40 & \text{Zirconium} & \text{Zr} & 91.224 \\ 41 & \text{Niobium} & \text{Nb} & 92.90638 \\ 42 & \text{Molybdenum} & \text{Mo} & 95.94 \\ 43 & \text{Technetium*} & \text{Tc}^{98} & 97.9072 \\ 44 & \text{Ruthenium} & \text{Ru} & 101.07 \\ 45 & \text{Rhodium} & \text{Rh} & 102.90550 \\ 46 & \text{Palladium} & \text{Pd} & 106.42 \\ 47 & \text{Silver} & \text{Ag} & 107.8682 \\ 48 & \text{Cadmium} & \text{Cd} & 112.411 \\ 49 & \text{Indium} & \text{In} & 114.818 \\ 50 & \text{Tin} & \text{Sn} & 118.710 \\ 51 & \text{Antimony} & \text{Sb} & 121.760 \\ 52 & \text{Tellurium} & \text{Te} & 127.60 \\ 53 & \text{Iodine} & \text{I} & 126.90447 \\ 54 & \text{Xenon} & \text{Xe} & 131.293 \\ 55 & \text{Cesium} & \text{Cs} & 132.90545 \\ 56 & \text{Barium} & \text{Ba} & 137.327 \\ 57 & \text{Lanthanum} & \text{La} & 138.9055 \\ 58 & \text{Cerium} & \text{Ce} & 140.116 \\ 59 & \text{Praseodymium} & \text{Pr} & 140.90765 \\ 60 & \text{Neodymium} & \text{Nd} & 144.24 \\ 61 & \text{Promethium} & \text{Pm}^{145} & 144.9127 \\ 62 & \text{Samarium} & \text{Sm} & 150.36 \\ 63 & \text{Europium} & \text{Eu} & 151.964 \\ 64 & \text{Gadolinium} & \text{Gd} & 157.25 \\ 65 & \text{Terbium} & \text{Tb} & 158.92534 \\ 66 & \text{Dysprosium} & \text{Dy} & 162.50 \\ 67 & \text{Holmium} & \text{Ho} & 164.93032 \\ 68 & \text{Erbium} & \text{Er} & 167.259 \\ 69 & \text{Thulium} & \text{Tm} & 168.93421 \\ 70 & \text{Ytterbium} & \text{Yb} & 173.04 \\ 71 & \text{Lutetium} & \text{Lu} & 174.967 \\ 72 & \text{Hafnium} & \text{Hf} & 178.49 \\ 73 & \text{Tantalum} & \text{Ta} & 180.9479 \\ 74 & \text{Tungsten} & \text{W} & 183.84 \\ 75 & \text{Rhenium} & \text{Re} & 186.207 \\ 76 & \text{Osmium} & \text{Os} & 190.23 \\ 77 & \text{Iridium} & \text{Ir} & 192.217 \\ 78 & \text{Platinum} & \text{Pt} & 195.078 \\ 79 & \text{Gold} & \text{Au} & 196.96655 \\ 80 & \text{Mercury} & \text{Hg} & 200.59 \\ 81 & \text{Thallium} & \text{Tl} & 204.3833 \\ 82 & \text{Lead} & \text{Pb} & 207.2 \\ 83 & \text{Bismuth} & \text{Bi} & 208.98038 \\ 84 & \text{Polonium*} & \text{Po}^{210} & 209.9829 \\ 85 & \text{Astatine*} & \text{At}^{210} & 209.9871 \\ 86 & \text{Radon*} & \text{Rn}^{222} & 222.0176 \\ 87 & \text{Francium*} & \text{Fr}^{223} & 223.0197 \\ 88 & \text{Radium*} & \text{Ra}^{226} & 226.0254 \\ 89 & \text{Actinium*} & \text{Ac}^{227} & 227.0277 \\ 90 & \text{Thorium*} & \text{Th} & 232.0381 \\ 91 & \text{Protactinium*} & \text{Pa} & 231.03588 \\ 92 & \text{Uranium*} & \text{U} & 238.02891 \\ 93 & \text{Neptunium*} & \text{Np}^{237} & 237.0482 \\ 94 & \text{Plutonium*} & \text{Pu}^{244} & 244.0642 \\ 95 & \text{Americium*} & \text{Am}^{243} & 243.0614 \\ 96 & \text{Curium*} & \text{Cm}^{247} & 247.0704 \\ 97 & \text{Berkelium*} & \text{Bk}^{247} & 247.0703 \\ 98 & \text{Californium*} & \text{Cf}^{251} & 251.0796 \\ 99 & \text{Einsteinium*} & \text{Es}^{252} & 252.0830 \\ 100 & \text{Fermium*} & \text{Fm}^{257} & 257.0951 \\ 101 & \text{Mendelevium*} & \text{Md}^{258} & 258.0984 \\ 102 & \text{Nobelium*} & \text{No}^{259} & 259.1010 \\ 103 & \text{Lawrencium*} & \text{Lr}^{262} & 262.1097 \\ 104 & \text{Rutherfordium*} & \text{Rf}^{261} & 261.1088 \end{array} \nonumber$

This table is slightly modified from that given in “ATOMIC WEIGHTS OF THE ELEMENTS, 1999” published by the International Union of Pure and Applied Chemistry, Inorganic Chemistry Division, Commission on Atomic Weights and Isotopic Abundances. Elements 105 – 118 are omitted. The Commission’s report was prepared for publication by T. B. Coplen, U.S. Geological Survey, 431 National Center, Reston, Virginia 20192, USA. See http://www.physics.curtin.edu.au/iupac/docs/Atwt1999.doc

*Element has no stable nuclides. Three such elements ($$\text{Th}$$, $$\text{Pa}$$, and $$\text{U}$$) have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. When the element symbol is listed with an atomic number, that isotope has the longest half-life and its atomic weight is tabulated.