The atomic mass of an element is not always a whole number because it takes into account the presence of different isotopes of that element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Beryllium (Be) has two stable isotopes: beryllium-9 (^9Be) and beryllium-10 (^10Be). The atomic mass of an element is the weighted average of the masses of its isotopes, taking into account their relative abundances.
The majority of naturally occurring beryllium is beryllium-9, which has a mass of approximately 9 atomic mass units (amu). However, a small fraction of beryllium consists of beryllium-10, which has a slightly higher mass. This means that the average atomic mass of beryllium is slightly higher than 9.
To calculate the atomic mass of beryllium, you would need to consider the abundance of each isotope. The atomic mass of 9.01 amu reflects the contribution of both isotopes to the overall average.