Mononuclidic elements

Mononuclidic elements are chemical elements which either have only one stable isotope, or for which there is only one very long-lived isotope that dominates the element's natural isotopic abundance. There are nineteen elements in the first category, and three ( Bi, Th and  Pa) in the second. Elements such as Rb, In, and La, that have a single stable isotope and a significant fraction of a very long-lived isotope in their natural abundance, are not considered mononuclidic.

Use in metrology
Mononuclidic elements are of scientific importance because their atomic weights can be measured to high accuracy, since there is minimal uncertainty associated with the isotopic abundances present in a given sample.

Contamination by unstable trace isotopes
Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, 10Be, with a half-life of 1.5 million years, is produced by cosmic rays in the Earth's upper atmosphere;  129I, with a half-life of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms;  137Cs, with a half-life of 30 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.

Complete list
Data from Atomic Weights and Isotopic Compositions ed. J. S. Coursey, D. J. Schwab and R. A. Dragoset, National Institute of Standards and Technology (2005).