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An element's atomic number indicates the number of protons it has in its nucleus. Beryllium's atomic number is 4. It has 4 protons. Every atom of beryllium contains exactly 4 protons. If an element has just one more, it is not beryllium (it is boron which has an atomic number of 5). The elements on the periodic table are arranged by increasing atomic number.
In neutral atoms, the atomic number also equals the number of electrons in an atom. Beryllium has 4 electrons. Another example:
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