The Atomic Number is equal to the amount of Protons and Electrons. To find the amount of Neutrons in an atom, you have to look at the Mass Number. The Mass Number is the SUM (_+_=_) of Protons and Neutrons in an atom. In this case, you will have to make up equations. For example: Argon. Argon's Mass Number is 40. You need to find the amount of Protons in the atom (18). Mass Number (40) - Protons (18) = Amount of Neutrons. 40-18=22.
Argon has 22 Neutrons, because Protons(18)+Neutrons(22)=Mass Number(40).
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