Question

what do you mean by the statement average atomic mass of an element x is 16.2

Answer 1

When an element exist with more than one form with some % abundance that can be contributed to the atomic mass, the atomic mass will be called as average atomic mass and at that time the mass is not a whole number.

$$\begin{gathered} AverageAtomicmass=\frac{X_1{M}_1+X_2{M}_2}{X_1+X_2} \\ Where \\ {X}_{1,}{X}_2-aretherelativeabundance\% \\ {M}_1,M_2-arethemassnumber. \end{gathered}$$
As the average atomic mass given is 16.2, it exist with more than one form, with different mass number - called isotope.