Protons are made of quarks. A proton is believed to be made up of 3 quarks, two up quarks and one down quark. One down quark has a charge of -1/3, and two up quarks have a charge of +2/3 each. This adds to a charge of +1. A proton has a very small mass. The mass of the proton is about one atomic mass unit. The mass of the neutron is also about one atomic mass unit. The size of a proton is determined by the vibration of the quarks that are in it, and these quarks effectively form a cloud. This means that a proton is not so much a hard ball as an area that contains quarks.