nistiso dataset

Masses, isotope patterns and nucleon numbers of the elements

A data frame giving the masses, standard isotopic abundances and nucleon numbers of most chemical elements as provided by the Physical Measurement Laboratory of the National Institute of Standards and Technology (but taken from separate publications). It includes the four variables element (the chemical symbol), mass, abundance and nucleons, arranged so that each isotope is uniquely specified by one row. data

References

http://www.nist.gov/pml/data/comp.cfm