The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D).  The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope.  (Atomic mass is also referred to as atomic weight, but the term “mass” is more accurate.)

For instance, it can be determined experimentally that neon consists of three isotopes:  neon-20 (with 10 protons and 10 neutrons in its nucleus) with a mass of 19.992 amu and an abundance of 90.48%, neon-21 (with 10 protons and 11 neutrons) with a mass of 20.994 amu and an abundance of 0.27%, and neon-22 (with 10 protons and 12 neutrons) with a mass of 21.991 amu and an abundance of 9.25%.  The average atomic mass of neon is thus:

 0.9048 × 19.992 amu = 18.09 amu 0.0027 × 20.994 amu = 0.057 amu 0.0925 × 21.991 amu = 2.03 amu 20.18 amu

The atomic mass is useful in chemistry when it is paired with the mole concept:  the atomic mass of an element, measured in amu, is the same as the mass in grams of one mole of an element.  Thus, since the atomic mass of iron is 55.847 amu, one mole of iron atoms would weigh 55.847 grams. The same concept can be extended to ionic compounds and molecules.  One formula unit of sodium chloride (NaCl) would weigh 58.44 amu (22.98977 amu for Na + 35.453 amu for Cl), so a mole of sodium chloride would weigh 58.44 grams. One molecule of water (H2O) would weigh 18.02 amu (2×1.00797 amu for H + 15.9994 amu for O), and a mole of water molecules would weigh 18.02 grams.

The original periodic table of the elements published by Dimitri Mendeleev in 1869 arranged the elements that were known at the time in order of increasing atomic weight, since this was prior to the discovery of the nucleus and the interior structure of the atom.  The modern periodic table is arranged in order of increasing atomic number instead.

### The Elements, sorted by Atomic Mass

 Atomic Number Symbol Name Atomic Weight (amu, g/mol) 1 H Hydrogen 1.00797 2 He Helium 4.00260 3 Li Lithium 6.941 4 Be Beryllium 9.01218 5 B Boron 10.81 6 C Carbon 12.011 7 N Nitrogen 14.0067 8 O Oxygen 15.9994 9 F Fluorine 18.998403 10 Ne Neon 20.179 11 Na Sodium 22.98977 12 Mg Magnesium 24.305 13 Al Aluminum 26.98154 14 Si Silicon 28.0855 15 P Phosphorus 30.97376 16 S Sulfur 32.06 17 Cl Chlorine 35.453 19 K Potassium 39.0983 18 Ar Argon 39.948 20 Ca Calcium 40.08 21 Sc Scandium 44.9559 22 Ti Titanium 47.90 23 V Vanadium 50.9415 24 Cr Chromium 51.996 25 Mn Manganese 54.9380 26 Fe Iron 55.847 28 Ni Nickel 58.70 27 Co Cobalt 58.9332 29 Cu Copper 63.546 30 Zn Zinc 65.38 31 Ga Gallium 69.72 32 Ge Germanium 72.59 33 As Arsenic 74.9216 34 Se Selenium 78.96 35 Br Bromine 79.904 36 Kr Krypton 83.80 37 Rb Rubidium 85.4678 38 Sr Strontium 87.62 39 Y Yttrium 88.9059 40 Zr Zirconium 91.22 41 Nb Niobium 92.9064 42 Mo Molybdenum 95.94 43 Tc Technetium (98) 44 Ru Ruthenium 101.07 45 Rh Rhodium 102.9055 46 Pd Palladium 106.4 47 Ag Silver 107.868 48 Cd Cadmium 112.41 49 In Indium 114.82 50 Sn Tin 118.69 51 Sb Antimony 121.75 53 I Iodine 126.9045 52 Te Tellurium 127.60 54 Xe Xenon 131.30 55 Cs Cesium 132.9054 56 Ba Barium 137.33 57 La Lanthanum 138.9055 58 Ce Cerium 140.12 59 Pr Praseodymium 140.9077 60 Nd Neodymium 144.24 61 Pm Promethium (145) 62 Sm Samarium 150.4 63 Eu Europium 151.96 64 Gd Gadolinium 157.25 65 Tb Terbium 158.9254 66 Dy Dysprosium 162.50 67 Ho Holmium 164.9304 68 Er Erbium 167.26 69 Tm Thulium 168.9342 70 Yb Ytterbium 173.04 71 Lu Lutetium 174.967 72 Hf Hafnium 178.49 73 Ta Tantalum 180.9479 74 W Tungsten 183.85 75 Re Rhenium 186.207 76 Os Osmium 190.2 77 Ir Iridium 192.22 78 Pt Platinum 195.09 79 Au Gold 196.9665 80 Hg Mercury 200.59 81 Tl Thallium 204.37 82 Pb Lead 207.2 83 Bi Bismuth 208.9804 84 Po Polonium (209) 85 At Astatine (210) 86 Rn Radon (222) 87 Fr Francium (223) 88 Ra Radium 226.0254 89 Ac Actinium 227.0278 91 Pa Protactinium 231.0359 90 Th Thorium 232.0381 93 Np Neptunium 237.0482 92 U Uranium 238.029 94 Pu Plutonium (242) 95 Am Americium (243) 97 Bk Berkelium (247) 96 Cm Curium (247) 102 No Nobelium (250) 98 Cf Californium (251) 99 Es Einsteinium (252) 108 Hs Hassium (255) 109 Mt Meitnerium (256) 100 Fm Fermium (257) 101 Md Mendelevium (258) 103 Lr Lawrencium (260) 104 Rf Rutherfordium (261) 107 Bh Bohrium (262) 105 Db Dubnium (262) 106 Sg Seaborgium (263) 110 Uun Ununnilium (269) 111 Uuu Unununium (272) 112 Uub Ununbiium (277) 113 — —— ——— 114 Uuq Ununquadium