sentence: Protons are made of quarks.
sentences_after: A proton is believed to be made up of 3 quarks, two up quarks and one down quark. 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.
concept_name: proton
headings: Proton
categories: Subatomic particles