The charge on a proton is positive and charge on an electron is negative. An atom contains three types of particles electron, proton and neutron. Neutron is electronically neutral. The charge on an atom is based upon the number of protons and number of electrons. If the number of proton and number of electrons become equal, they will cancel the effect of each other. So, an atom is neutral when number of protons and number of electrons are equal. Moreover, when an atom loses or gains electron ions are formed.
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