why do atoms have no overall charge

Atoms have no overall charge because they contain equal numbers of positively charged protons and negatively charged electrons. The positive charge of the protons (+1 each) is exactly balanced by the negative charge of the electrons (-1 each), so these charges cancel out, resulting in a neutral atom with no net charge. Neutrons, which also reside in the nucleus along with protons, have no charge and therefore do not affect the atom's overall charge.