# Why is the atomic mass of an element usually fractional?

The majority of elements exist as a mixture of isotopes of different masses. Because of this combination, fractional atomic masses arise. The atomic mass is the average of atomic masses of all the isotopes of an element, and it can be expressed as,

$$Average\, mass = \frac{Total\, mass\, of\, all\, atoms}{number\, of\, atoms}$$