The net electric charge of the nucleus of an atom is positive due to the presence of charge-carrying particles known as protons. The outer shell of the atom is negatively charged as a result of carrying electrons in a diffuse cloud. Atoms always have a net neutral electric charge.
Protons all carry positive charges, so the electromagnetic force tends to drive them away from each other. Like charges repel each other and must be overcome by another, stronger force. This is why the nucleus of almost all atoms contains neutrons, which have no charge of their own, but exchange strong force-carrying particles called mesons that bind the nucleus together.