In the Standard Model of physics, electrons are truly elementary particles with no internal structure. However, both protons and neutrons are composite particles composed of elementary particles called quarks. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two up quarks and one down quark. Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles.