IUPAC Commission on Isotopic Abundances and Atomic Weights.

ATOMIC WEIGHTS OF THE ELEMENTS 2021

These tables are based on the 2021 table with changes from the 2019 table for the values of Ar, Hf, Ir, Pb and Yb and changes to the uncertainty for Al, Au, Co, F, Ho, Mn, Nb, Pa, Pr, Rh, Sc, Tb, Tm, and Y.

https://iupac.qmul.ac.uk/AtWt/

World Wide Web version of atomic weight data originally prepared by G. P. Moss, from a file provided by D. R. Lide.

Previous values may be consulted from the 1993 table, the 1995 table, the 1997 table, the 1999 table, the 2001 table, the 2005 table, the 2007 table, the 2009 table, the 2011 table, the 2013 table, the 2015 table or the 2019 table.

The standard atomic weights of twelve elements having two or more stable isotopes have variability of atomic-weight values in natural terrestrial materials. These are given in table 1 below. In the other lists the values quoted are those suggested for material where the origin of the sample is unknown. For radioactive elements the isotope with the longest half-life is quoted in parenthesis. The original paper should be consulted for full details of the variation in atomic weight and the half life of the radioisotopes quoted below.

A number in parentheses indicates the uncertainty in the last digit of the atomic weight.

See below for the elements listed in Atomic Number Order or Name order.

See also a copy of the periodic table with atomic weights to five significant figures.

Table 1. List of Elements with Range of Atomic Weights.

At No Symbol Name Minimum Atomic Wt Maximum Atomic Wt

1 H hydrogen 1.007 84 1.008 11

3 Li lithium 6.938 6.997

5 B boron 10.806 10.821

6 C carbon 12.0096 12.0116

7 N nitrogen 14.006 43 14.007 28

8 O oxygen 15.999 03 15.999 77

12 Mg magnesium 24.304 24.307

14 Si silicon 28.084 28.086

16 S sulfur 32.059 32.076

17 Cl chlorine 35.446 35.457

18 Ar argon 39.792 39.963

35 Br bromine 79.901 79.907

81 Tl thallium 204.382 204.385

82 Pb lead 206.14 207.94

See original paper for the range of these elements from different sources [Isotope-abundance variations and atomic weights of selected elements: 2016 (IUPAC Technical Report), Pure Appl. Chem. 2016, 88(12), 1203-1224., Pure Appl. Chem. 2021, 93(1), 155-166 and Pure Appl. Chem. 2022, 94]

Table 2. List of Elements in Atomic Number Order.

At No Symbol Name Atomic Wt Notes

1 H Hydrogen 1.008 3, 5

2 He Helium 4.002 602(2) 1, 2

3 Li Lithium 6.94 3, 5

4 Be Beryllium 9.012 1831(5)

5 B Boron 10.81 3, 5

6 C Carbon 12.011 5

7 N Nitrogen 14.007 5

8 O Oxygen 15.999 5

9 F Fluorine 18.998 403 16251)

10 Ne Neon 20.1797(6) 1, 3

11 Na Sodium 22.989 769 28(2)

12 Mg Magnesium 24.305 5

13 Al Aluminium 26.981 5384(3)

14 Si Silicon 28.085 5

15 P Phosphorus 30.973 761 998(5)

16 S Sulfur 32.06 5

17 Cl Chlorine 35.45 3, 5

18 Ar Argon 39.95(16) 1, 2, 5

19 K Potassium 39.0983(1)

20 Ca Calcium 40.078(4)

21 Sc Scandium 44.955 907(4)

22 Ti Titanium 47.867(1)

23 V Vanadium 50.9415(1)

24 Cr Chromium 51.9961(6)

25 Mn Manganese 54.938 043(2)

26 Fe Iron 55.845(2)

27 Co Cobalt 58.933 194(3)

28 Ni Nickel 58.6934(4) 2

29 Cu Copper 63.546(3) 2

30 Zn Zinc 65.38(2) 2

31 Ga Gallium 69.723(1)

32 Ge Germanium 72.630(8)

33 As Arsenic 74.921 595(6)

34 Se Selenium 78.971(8)

35 Br Bromine 79.904 5

36 Kr Krypton 83.798(2) 1, 3

37 Rb Rubidium 85.4678(3) 1

38 Sr Strontium 87.62(1) 1, 2

39 Y Yttrium 88.905 838(2)

40 Zr Zirconium 91.224(2) 1

41 Nb Niobium 92.906 37(1)

42 Mo Molybdenum 95.95(1) 1

43 Tc Technetium [97] 4

44 Ru Ruthenium 101.07(2) 1

45 Rh Rhodium 102.905 49(2)

46 Pd Palladium 106.42(1) 1

47 Ag Silver 107.8682(2) 1

48 Cd Cadmium 112.414(4) 1

49 In Indium 114.818(1)

50 Sn Tin 118.710(7) 1

51 Sb Antimony 121.760(1) 1

52 Te Tellurium 127.60(3) 1

53 I Iodine 126.904 47(3)

54 Xe Xenon 131.293(6) 1, 3

55 Cs Caesium 132.905 451 96(6)

56 Ba Barium 137.327(7)

57 La Lanthanum 138.905 47(7) 1

58 Ce Cerium 140.116(1) 1

59 Pr Praseodymium 140.907 66(1)

60 Nd Neodymium 144.242(3) 1

61 Pm Promethium [145]

62 Sm Samarium 150.36(2) 1

63 Eu Europium 151.964(1) 1

64 Gd Gadolinium 157.25(3) 1

65 Tb Terbium 158.925 354(7)

66 Dy Dysprosium 162.500(1) 1

67 Ho Holmium 164.930 329(5)

68 Er Erbium 167.259(3) 1

69 Tm Thulium 168.934 219(5)

70 Yb Ytterbium 173.045(10) 1

71 Lu Lutetium 174.966 8(1) 1

72 Hf Hafnium 178.486(6)

73 Ta Tantalum 180.947 88(2)

74 W Tungsten 183.84(1)

75 Re Rhenium 186.207(1)

76 Os Osmium 190.23(3) 1

77 Ir Iridium 192.217(2)

78 Pt Platinum 195.084(9)

79 Au Gold 196.966 570(4)

80 Hg Mercury 200.592(3)

81 Tl Thallium 204.38 5

82 Pb Lead 207.2(1) 1, 2, 5

83 Bi Bismuth 208.980 40(1)

84 Po Polonium [209] 4

85 At Astatine [210] 4

86 Rn Radon [222] 4

87 Fr Francium [223] 4

88 Ra Radium [226] 4

89 Ac Actinium [227] 4

90 Th Thorium 232.0377(4) 1, 4

91 Pa Protactinium 231.035 88(1) 4

92 U Uranium 238.028 91(3) 1, 3, 4

93 Np Neptunium [237] 4

94 Pu Plutonium [244] 4

95 Am Americium [243] 4

96 Cm Curium [247] 4

97 Bk Berkelium [247] 4

98 Cf Californium [251] 4

99 Es Einsteinium [252] 4

100 Fm Fermium [257] 4

101 Md Mendelevium [258] 4

102 No Nobelium [259] 4

103 Lr Lawrencium [262] 4

104 Rf Rutherfordium [267] 4

105 Db Dubnium [270] 4

106 Sg Seaborgium [269] 4

107 Bh Bohrium [270] 4

108 Hs Hassium [270] 4

109 Mt Meitnerium [278] 4

110 Ds Darmstadtium [281] 4

111 Rg Roentgenium [281] 4

112 Cn Copernicium [285] 4

113 Nh Nihonium [286] 4

114 Fl Flerovium [289] 4

115 Mc Moscovium [289] 4

116 Lv Livermorium [293] 4

117 Ts Tennessine [293] 4

118 Og Oganesson [294] 4

Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the Table may exceed the stated uncertainty.
Range in isotopic composition of normal terrestrial material prevents a more precise value being given; the tabulated value should be applicable to any normal material.
Modified isotopic compositions may be found in commercially available material because it has been subject to an undisclosed or inadvertant isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the Table can occur.
Element has no stable nuclides. The value enclosed in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated.
See table 1 for details of range and original paper for the atomic weight of the element from different sources.

Table 3. List of Elements in Name Order.

At No Symbol Name Atomic Wt Notes

89 Ac Actinium [227] 4

13 Al Aluminium 26.981 5384(3)

95 Am Americium [243] 4

51 Sb Antimony 121.760(1) 1

18 Ar Argon 39.95(16) 1, 2, 5

33 As Arsenic 74.921 595(6)

85 At Astatine [210] 4

56 Ba Barium 137.327(7)

97 Bk Berkelium [247] 4

4 Be Beryllium 9.012 1831(5)

83 Bi Bismuth 208.980 40(1)

107 Bh Bohrium [270] 4

5 B Boron 10.81 3, 5

35 Br Bromine 79.904 5

48 Cd Cadmium 112.414(4) 1

55 Cs Caesium 132.905 451 96(6)

20 Ca Calcium 40.078(4) 1

98 Cf Californium [251] 4

6 C Carbon 12.011 5

58 Ce Cerium 140.116(1) 1

17 Cl Chlorine 35.45 3, 5

24 Cr Chromium 51.9961(6)

27 Co Cobalt 58.933 194(3)

112 Cn Copernicium [285] 4

29 Cu Copper 63.546(3) 2

96 Cm Curium [247] 4

110 Ds Darmstadtium [281] 4

105 Db Dubnium [270] 4

66 Dy Dysprosium 162.500(1) 1

99 Es Einsteinium [252] 4

68 Er Erbium 167.259(3) 1

63 Eu Europium 151.964(1) 1

100 Fm Fermium [257] 4

114 Fl Flerovium [289] 4

9 F Fluorine 18.998 403 162(5)

87 Fr Francium [223] 4

64 Gd Gadolinium 157.25(3) 1

31 Ga Gallium 69.723(1)

32 Ge Germanium 72.630(8)

79 Au Gold 196.966 570(4)

72 Hf Hafnium 178.486(6)

108 Hs Hassium [270] 4

2 He Helium 4.002 602(2) 1, 2

67 Ho Holmium 164.930 329(5)

1 H Hydrogen 1.008 3, 5

49 In Indium 114.818(1)

53 I Iodine 126.904 47(3)

77 Ir Iridium 192.217(2)

26 Fe Iron 55.845(2)

36 Kr Krypton 83.798(2) 1, 3

57 La Lanthanum 138.905 47(7) 1

103 Lr Lawrencium [262] 4

82 Pb Lead 207.2(1) 1, 2, 5

3 Li Lithium 6.94 3, 5

116 Lv Livermorium [293] 4

71 Lu Lutetium 174.9668(1) 1

12 Mg Magnesium 24.305 5

25 Mn Manganese 54.938 043(2)

109 Mt Meitnerium [278] 4

101 Md Mendelevium [258] 4

80 Hg Mercury 200.592(3)

42 Mo Molybdenum 95.95(1) 1

115 Mc Moscovium [289] 4

60 Nd Neodymium 144.242(3) 1

10 Ne Neon 20.1797(6) 1, 3

93 Np Neptunium [237] 4

28 Ni Nickel 58.6934(4)

113 Nh Nihonium [286] 4

41 Nb Niobium 92.906 37(1)

7 N Nitrogen 14.007 5

102 No Nobelium [259] 4

118 Og Oganesson [294] 4

76 Os Osmium 190.23(3) 1

8 O Oxygen 15.999 5

46 Pd Palladium 106.42(1) 1

15 P Phosphorus 30.973 761 998(5)

78 Pt Platinum 195.084(9)

94 Pu Plutonium [244] 4

84 Po Polonium [209] 4

19 K Potassium 39.0983(1)

59 Pr Praseodymium 140.907 66(1)

61 Pm Promethium [145] 4

91 Pa Protactinium 231.035 88(1) 4

88 Ra Radium [226] 4

86 Rn Radon [222] 4

75 Re Rhenium 186.207(1)

45 Rh Rhodium 102.905 49(2)

111 Rg Roentgenium [281] 4

37 Rb Rubidium 85.4678(3) 1

44 Ru Ruthenium 101.07(2) 1

104 Rf Rutherfordium [267] 4

62 Sm Samarium 150.36(2) 1

21 Sc Scandium 44.955 907(4)

106 Sg Seaborgium [269] 4

34 Se Selenium 78.971(8)

14 Si Silicon 28.085 5

47 Ag Silver 107.8682(2) 1

11 Na Sodium 22.989 769 28(2)

38 Sr Strontium 87.62(1) 1, 2

16 S Sulfur 32.06 5

73 Ta Tantalum 180.947 88(2)

43 Tc Technetium [97] 4

52 Te Tellurium 127.60(3) 1

117 Ts Tennessine [293] 4

65 Tb Terbium 158.925 354(7)

81 Tl Thallium 204.38 5

90 Th Thorium 232.0377(4) 1, 4

69 Tm Thulium 168.934 219(5)

50 Sn Tin 118.710(7) 1

22 Ti Titanium 47.867(1)

74 W Tungsten 183.84(1)

92 U Uranium 238.028 91(3) 1, 3, 4

23 V Vanadium 50.9415(1)

54 Xe Xenon 131.293(6) 1, 3

70 Yb Ytterbium 173.045(10) 1

39 Y Yttrium 88.905 838(2)

30 Zn Zinc 65.38(2) 2

40 Zr Zirconium 91.224(2) 1

Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the Table may exceed the stated uncertainty.
Range in isotopic composition of normal terrestrial material prevents a more precise value being given; the tabulated value should be applicable to any normal material.
Modified isotopic compositions may be found in commercially available material because it has been subject to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the Table can occur.
Element has no stable nuclides. The value enclosed in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated.
See table 1 for details of range and original paper for the atomic weight of the element from different sources.

Return to IUPAC Chemical Nomenclature home page

At No	Symbol	Name	Minimum Atomic Wt	Maximum Atomic Wt
1	H	hydrogen	1.007 84	1.008 11
3	Li	lithium	6.938	6.997
5	B	boron	10.806	10.821
6	C	carbon	12.0096	12.0116
7	N	nitrogen	14.006 43	14.007 28
8	O	oxygen	15.999 03	15.999 77
12	Mg	magnesium	24.304	24.307
14	Si	silicon	28.084	28.086
16	S	sulfur	32.059	32.076
17	Cl	chlorine	35.446	35.457
18	Ar	argon	39.792	39.963
35	Br	bromine	79.901	79.907
81	Tl	thallium	204.382	204.385
82	Pb	lead	206.14	207.94