Standard atomic masses of the chemical elements, in unified atomic mass units (equivalent to g/mol) with uncertainties in shorthand form, ordered by atomic number

Atomic mass

Notes

• g: Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic mass of the element in such specimens and that given in the table may exceed the stated uncertainty.
• l: Commercially available lithium materials have atomic masses that range between 6.939–6.996; if a more accurate value is required, it must be determined for the specific material.
• m: 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 mass of the element from that given in the table can occur.
• n: Element has no stable nuclides. The value 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 mass is tabulated.
• r: 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.

References

• Atomic Weights of the Elements 2001, Pure Appl. Chem. 75(8), 1107-1122, 2003