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Monoisotopic elements

In normal materials, there are nineteen (19) elements whose standard atomic weight is determined by only one isotope, which is also stable (non-radioactive).note Thus, the standard atomic weight for these elements is invariant. These elements are: Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, and Au.
In addition, two elements, Bi and Pa, have only one isotope that contributes to the standard atomic weight, but that isotope is radioactive.

Standard Atomic Weights 2021

Z Symbol Element Standard
Atomic Weight
 4Beberyllium  9.012 1831(5)
 9Ffluorine 18.998 403 162(5)
11Nasodium 22.989 769 28(2)
13Alaluminium 26.981 5384(3)
15Pphosphorus 30.973 761 998(5)
21Scscandium 44.955 907(4)
25Mnmanganese 54.938 043(2)
27Cocobalt 58.933 194(3)
33Asarsenic 74.921 595(6)
39Yyttrium 88.905 838(2)
41Nbniobium 92.906 37(1)
45Rhrhodium102.905 49(2)
53Iiodine126.904 47(3)
55Cscaesium132.905 451 96(6)
59Prpraseodymium140.907 66(1)
65Tbterbium158.925 354(7)
67Hoholmium164.930 329(5)
69Tmthulium168.934 219(5)
79Augold196.966 570(4)
83Bibismuth208.980 40(1)
91Paprotactinium231.035 88(1)
Z Symbol Element Standard
Atomic Weight

Note      Back to Top
There are additional five elements (V, Rb, In, La, and Lu) that have only one stable isotope, but all of these elements have a second, radioactive, isotope which contributes significantly to the standard atomic weight.

The standard atomic weights of these 19+2 elements are derived directly from their atomic masses, which are evaluated and published by IUPAP.

For atomic masses of all other stable nuclides see the Current Atomic Masses


The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2021" report.