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: Gold has an atomic number of 79. A neutral atom of gold has 79 protons and 79 electrons.
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