Here's why:

* Mass Number: The mass number of an atom is the total number of protons and neutrons in its nucleus.

* Beryllium's Atomic Number: Beryllium's atomic number is 4, which means it has 4 protons.

* Beryllium's Number of Neutrons: Beryllium's most common isotope has 5 neutrons (although other isotopes exist).

Therefore, 4 protons + 5 neutrons = 9 (mass number).