Because that is the number of protons in its nucleus. This is also equal to the number of electrons in its orbits.
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The atomic number for beryllium is 4.
The atomic number of Beryllium is 4.
Beryllium Atomic number: 4 Atomic Mass: 9.012
The atomic number of beryllium (Be) is 4. The atomic weight of Be is 9.012182 grams per mole.See the Web Links to the left of this answer for a periodic table with more information about this element!
Beryllium has an atomic number of 4, which indicates the number of protons in its nucleus. Its atomic mass of 9 includes the total number of protons and neutrons in its nucleus. This means that beryllium has 4 protons and usually 5 neutrons.