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STANDARD ATOMIC WEIGHTS

Standard Atomic Weights1
Based on the assigned relative mass of 12C = 12. For the sake of completeness, all known elements are included in the list. Several of those more recently discovered are represented only by the unstable isotopes. In each case, the values in parentheses in the atomic weight column are the mass numbers of the most stable isotopes.

Name

Symbol
Atomic
No.
Atomic
Weight

Valence
Actinium Ac 89 227.028 . . .
Aluminum Al 13 26.9815 3
Americium Am 95 (243) 3,4,5,6
Antimony (stibium) Sb 51 121.75 3,5
Argon Ar 18 39.948 0
Arsenic As 33 74.9216 3,5
Astatine At 85 (210) 1,3,5,7
Barium Ba 56 137.33 2
Berkelium Bk 97 (247) 3,4
Beryllium Be 4 9.0122 2
Bismuth Bi 83 208.980 3,5
Boron B 5 10.81 3
Bromine Br 35 79.904 1,3,5,7
Cadmium Cd 48 112.41 2
Calcium Ca 20 40.08 2
Californium Cf 98 (251) . . .
Carbon C 6 12.011 2,4
Cerium Ce 58 140.12 3,4
Cesium Cs 55 132.9054 1
Chlorine Cl 17 35.453 1,3,5,7
Chromium Cr 24 51.996 2,3,6
Cobalt Co 27 58.9332 2,3
Columbium (see Niobium)      
Copper Cu 29 63.546 1,2
Curium Cm 96 (247) 3
Dysprosium Dy 66 162.50 3
Einsteinium Es 99 (252) . . .
Erbium Er 68 167.26 3
Europium Eu 63 151.96 2,3
Fermium Fm 100 (257) . . .
Fluorine F 9 18.9984 1
Francium Fr 87 (223) 1
Gadolinium Gd 64 157.25 3
Gallium Ga 31 69.72 2,3
Germanium Ge 32 72.59 4
Gold (aurum) Au 79 196.967 1,3
Hafnium Hf 72 178.49 4
Helium He 2 4.0026 0
Holmium Ho 67 164.930 3
Hydrogen H 1 1.0079 1
Indium In 49 114.82 3
Iodine I 53 126.905 1,3,5,7
Iridium Ir 77 192.22 3,4
Iron (ferrum) Fe 26 55.847 2,3
Krypton Kr 36 83.80 0
Lanthanum La 57 138.906 3
Lawrencium Lr 103 (260) . . .
Lead (plumbum) Pb 82 207.2 2,4
Lithium Li 3 6.941 1
Lutetium Lu 71 174.967 3
Magnesium Mg 12 24.305 2
Manganese Mn 25 54.9380 2,3,4,6,7
Mendelevium Md 101 (258) . . .
Mercury (hydrargyrum) Hg 80 200.59 1,2
Molybdenum Mo 42 95.94 3,4,6
Neodymium Nd 60 144.24 3
Neon Ne 10 20.1179 0
Neptunium Np 93 237.0482 4,5,6
Nickel Ni 28 58.69 2,3
Niobium (columbium) Nb 41 92.9064 3,5
Nitrogen N 7 14.0067 3,5
Nobelium No 102 (259) . . .
Osmium Os 76 190.2 2,3,4,8
Oxygen O 8 15.9994 2
Palladium Pd 46 106.42 2,4,6
Phosphorus P 15 30.9738 3,5
Platinum Pt 78 195.08 2,4
Plutonium Pu 94 (244) 3,4,5,6
Polonium Po 84 (209) . . .
Potassium (kalium) K 19 39.0983 1
Praseodymium Pr 59 140.908 3
Promethium Pm 61 (145) 3
Protoactinium Pa 91 231.0359 . . .
Radium Ra 88 226.025 2
Radon Rn 86 (222) 0
Rhenium Re 75 186.207 . . .
Rhodium Rh 45 102.906 3
Rubidium Rb 37 85.4678 1
Ruthenium Ru 44 101.07 3,4,6,8
Samarium Sm 62 150.36 2,3
Scandium Sc 21 44.9559 3
Selenium Se 34 78.96 2,4,6
Silicon Si 14 28.0855 4
Silver (argentum) Ag 47 107.868 1
Sodium (natrium) Na 11 22.9898 1
Strontium Sr 38 87.62 2
Sulfur S 16 32.06 2,4,6
Tantalum Ta 73 180.9479 5
Technetium Tc 43 (98) 6,7
Tellurium Te 52 127.60 2,4,6
Terbium Tb 65 158.925 3
Thallium Tl 81 204.383 1,3
Thorium Th 90 232.038 4
Thulium Tm 69 168.934 3
Tin (stannum) Sn 50 118.71 2,4
Titanium Ti 22 47.88 3,4
Tungsten (wolfram) W 74 183.85 6
Uranium U 92 238.029 4,6
Vanadium V 23 50.9415 3,5
Xenon Xe 54 131.29 0
Ytterbium Yb 70 173.04 2,3
Yttrium Y 39 88.9059 3
Zinc Zn 30 65.39 2
Zirconium Zr 40 91.224 4
1 Modified and reproduced, with permission from Lide DR (editor-in-chief): CRC Handbook of Chemistry and Physics, 83rd ed. CRC Press, 2002-2003.
 

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