Atom How do you determine the number of electrons, protons, neutrons in an atom?

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I concur to previous posts.  Each element on the periodic chart has its own square.  At the top of the square is a number called the atomic number, which indicates the number of protons in the atom.  The number of protons is also equal to the number of electrons.  At the bottom of the square is a number called the mass number, which is the weighted average of all known isotopes of the atom.  Subtract the atomic number from this, you get the number of neutrons.

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