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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.

Z Symbol Element Standard
Atomic Weight
 4Beberyllium  9.012 1831(5)
 9Ffluorine 18.998 403 163(6)
11Nasodium 22.989 769 28(2)
13Alaluminium 26.981 5385(7)
15Pphosphorus 30.973 761 998(5)
21Scscandium 44.955 908(5)
25Mnmanganese 54.938 044(3)
27Cocobalt 58.933 194(4)
33Asarsenic 74.921 595(6)
39Yyttrium 88.905 84(2)
41Nbniobium 92.906 37(2)
45Rhrhodium102.905 50(2)
53Iiodine126.904 47(3)
55Cscaesium132.905 451 96(6)
59Prpraseodymium140.907 66(2)
65Tbterbium158.925 35(2)
67Hoholmium164.930 33(2)
69Tmthulium168.934 22(2)
79Augold196.966 569(5)
83Bibismuth208.980 40(1)
91Paprotactinium231.035 88(2)
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