The atomic mass of an element is determined by the average mass of its isotopes, taking into account their relative abundance. Beryllium (Be) has two naturally occurring isotopes: beryllium-9 (9Be) and beryllium-10 (10Be).
The atomic mass of an element is calculated by multiplying the mass of each isotope by its abundance, and then summing up the contributions from all the isotopes. The atomic mass of beryllium is 9.01 amu (atomic mass units) because it is closer in value to the weighted average of the masses of the two isotopes.
The majority of beryllium atoms (over 99%) have a mass of 9 atomic mass units (9Be), while a small fraction (less than 1%) have a mass of 10 atomic mass units (10Be). Since the mass of 9Be is closer to 9, but not exactly 9, and its abundance is significantly higher, it contributes more to the weighted average and influences the atomic mass of beryllium.
The presence of the heavier isotope, 10Be, which has a mass of 10, slightly increases the average atomic mass of beryllium above 9. If beryllium consisted solely of the 9Be isotope, its atomic mass would indeed be exactly 9. However, the presence of the less abundant 10Be isotope leads to a slightly higher average atomic mass of 9.01.