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Excellent! As you can see in the figure, there are a total of \(4\) electrons in the Beryllium atom. So, its atomic number is \(4\).
The atomic number doesn't depend on the number of neutrons. For that, we need the Mass Number. The mass number is just the number of protons plus the number of neutrons

\(\text{Mass Number}\) = \(\text{No. of protons}\) \(+ \, \text{No. of neutrons}\)

For example, as there are \(4\) protons and \(5\) neutrons in Beryllium, Beryllium's mass number is \(4+5=9\). It is called the Mass Number because the mass of an atom is determined by the number of protons and neutrons in it. The electrons have a much smaller mass, so, their contribution is negligible.