Their properties and interactions are described by nuclear physics. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Protons and neutrons constitute the nuclei of atoms. , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. The neutron is a subatomic particle, symbol < 2.9 ×10 −26 e⋅cm (experimental upper limit) Forces between quarks are mediated by gluons. The color assignment of individual quarks is arbitrary, but all three colors must be present.
