And the reason they don't is because of the strong nuclear force. In physics, there are four fundamental forces: gravity, the electromagnetic force, the weak force, and the strong force. The strong force is, unsurprisingly, strong. The strong force is the force that holds the nucleus of the atom together, even through other forces want to pull it apart. The electrostatic repulsion (part of the electromagnetic force) between the positive protons in the nucleus is very powerful.