What does the number 9 represent Be-9?

The number 9 in Be-9 represents the Atomic Mass number of the isotope beryllium-9. This indicates that the isotope has a total of 9 nucleons, which includes 4 protons and 5 neutrons. Beryllium itself has an atomic number of 4, signifying that it has 4 protons in its nucleus. Thus, Be-9 is the most stable and abundant isotope of beryllium.