Final answer:
An atom has an overall charge of zero when the number of protons (+1 charge) balances the number of electrons (-1 charge).
Step-by-step explanation:
An atom is made up of three subatomic particles: protons, neutrons, and electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. In a neutral atom, the number of protons is equal to the number of electrons, resulting in a charge of zero. For example, if an atom has two protons, it must also have two electrons to maintain a neutral charge.
So therefore an atom has an overall charge of zero because the number of protons (+1 charge) balances out the number of electrons (-1 charge). Each electron carries a negative charge of -1, while each proton carries a positive charge of +1. In order for the charges to cancel out and create a neutral atom, the number of protons must be equal to the number of electrons.