# What are the units for isotopic abundance?

Jan 19, 2017

These are reported as percentages..........

#### Explanation:

Some few nuclides occur as a single isotope, i.e. they are 100% abundant. Examples include ""^9Be, ""^31P, ""^55Mn, ""^209Bi.

The vast majority of nuclides, especially the transition metals, occur as a mixture of isotopes, where the atomic mass quoted on the Periodic Table, is the weighted average of the individual isotopes.

For example, $\text{iron}$, $Z = 26$ has an atomic mass of $55.8 \cdot g \cdot m o {l}^{-} 1$. This is the weighted average of the following isotopes: 5.845%, ""^54Fe; 91.754%, ""^56Fe; 2.119%, ""^57Fe; and 0.282%, ""^58Fe. If you took the exact mass of each isotope (which is not quite the quoted isotopic mass), you would get the atomic mass quoted on the Periodic Table.

Isotopes differ with respect to the number of which fundamental atomic particle?