The nucleus is the center of an atom. It is made up of nucleons called (protons and neutrons) and is surrounded by the electron cloud. The size (diameter) of the nucleus is between 1.6 fm (10 m) (for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium). These sizes are much smaller than the size of the atom itself by a factor of about 23,000 (uranium) to about 145,000 (hydrogen). Although it is only a very small part of the atom, the nucleus has most of the mass. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus with a very small contribution from the orbiting electrons.