2.1.5 Calculate the number of protons, neutrons, and electrons in atoms and ions from the mass number, atomic number, and charge.

• number of protons = number of electrons = Z

• number of neutrons = mass number - number of protons = A-Z

• number of electrons = number of protons - charge  of atom. 

Consider an atom of Sodium.



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• number of protons = number of electrons = 11 = Z

• number of neutrons = mass number - 11 = A-Z = 23 - 11 = 12

• number of electrons = 11 - (+1) = 10

Ions

loses electrons = positive charge = cation

gains electrons = negatively charged = anion