A proton is made of two up quarks and one down quark: uud.charge = (+2⁄3) + (+2⁄3) + (−1⁄3) = +1e A neutron is made of one up quark and two down quarks: udd.charge = (+2⁄3) + (−1⁄3) + (−1⁄3) = 0 The three quarks in each nucleon are held together by the strong nuclear force.The quark model helps explain why the proton and neutron have almost the same mass (both about 1.67 × 10−27 kg): each contains three quarks, and up and down quarks have similar, small masses. Most of a nucleon's mass actually comes from the energy of the strong interaction binding the quarks together.The antiparticles are made of the corresponding antiquarks:• antiproton p̄ = ūūd̄: charge (−2⁄3) + (−2⁄3) + (+1⁄3) = −1e
• antineutron n̄ = ūd̄d̄: charge 0Although the neutron has zero overall charge, it is made of charged quarks. Changing one quark into another changes a neutron into a proton or a proton into a neutron, which is the basis of beta decay.Worked example: a helium-4 nucleus contains 2 protons and 2 neutrons. How many up and down quarks does it contain?
up quarks = (2 × 2) + (2 × 1) = 6
down quarks = (2 × 1) + (2 × 2) = 6
• antineutron n̄ = ūd̄d̄: charge 0Although the neutron has zero overall charge, it is made of charged quarks. Changing one quark into another changes a neutron into a proton or a proton into a neutron, which is the basis of beta decay.Worked example: a helium-4 nucleus contains 2 protons and 2 neutrons. How many up and down quarks does it contain?
up quarks = (2 × 2) + (2 × 1) = 6
down quarks = (2 × 1) + (2 × 2) = 6