The electrical neutrality of the atom is achieved by the fact that the negative charge from the sum of the electrons is compensated by the positive charge of the same number of protons in the nucleus. Since neutrons, because of their vanishingly small gravitation in comparison to the electromagnetic repulsion of the protons among themselves could never overcome this repulsion, the nuclear building blocks, the nucleons, have to be kept together by much stronger forces. Thus there must be another natural force besides gravitation and electromagnetic interaction—the strong interaction.