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 |
|---|---|---|---|
| 4 | Be | beryllium | 9.012 1831(5) |
| 9 | F | fluorine | 18.998 403 162(5) |
| 11 | Na | sodium | 22.989 769 28(2) |
| 13 | Al | aluminium | 26.981 5384(3) |
| 15 | P | phosphorus | 30.973 761 998(5) |
| 21 | Sc | scandium | 44.955 907(4) |
| 25 | Mn | manganese | 54.938 043(2) |
| 27 | Co | cobalt | 58.933 194(3) |
| 33 | As | arsenic | 74.921 595(6) |
| 39 | Y | yttrium | 88.905 838(2) |
| 41 | Nb | niobium | 92.906 37(1) |
| 45 | Rh | rhodium | 102.905 49(2) |
| 53 | I | iodine | 126.904 47(3) |
| 55 | Cs | caesium | 132.905 451 96(6) |
| 59 | Pr | praseodymium | 140.907 66(1) |
| 65 | Tb | terbium | 158.925 354(7) |
| 67 | Ho | holmium | 164.930 329(5) |
| 69 | Tm | thulium | 168.934 219(5) |
| 79 | Au | gold | 196.966 570(4) |
| 83 | Bi | bismuth | 208.980 40(1) |
| 91 | Pa | protactinium | 231.035 88(1) |
| Z | Symbol | Element | Standard Atomic Weight |
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
Citation
The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2021" report.
