1. The relative amount of a given element among others; for example, the abundance of oxygen in the Earth’s crust is approximately 50% by mass.
2. The amount of a nuclide (stable or radioactive) relative to other nuclides of the same element in a given sample. The natural abundance is the abundance of a nuclide as it occurs naturally. For instance, chlorine has two stable isotopes of masses 35 and 37. The abundance of ³⁵Cl is 75.5% and that of ³⁷Cl is 24.5%. For some elements the abundance of a particular nuclide depends on the source.