According to experimental evidence and the discoveries of scientists like Ernest Rutherford, the nucleus of an atom contains the majority of its mass, about 99.97%. It is made up of protons, which have a positive charge, and neutrons, which have no charge. The nucleus is very small in comparison to the overall size of an atom, taking up only a tiny fraction of the atomic volume. It is the combination of protons and neutrons in the nucleus that determines an atom's atomic mass. The atomic mass is measured in atomic mass units, or daltons, and the contribution of the nucleus is usually around 99.97%.