It is now believed that protons and neutrons (which constitute nuclei of ordinary matter) are themselves built out of more elementary units called quarks. A proton and a neutron consist of three quarks each. Two types of quarks, the so called 'up' quark (denoted by u) of charge + (2/3)e and the 'down' quark (denoted by d) of charge (-1/3) e, together with electrons build up ordinary matter. (Quarks of other types have also been found which give rise to different unusual varieties of matter.) Suggest a possible quark composition of a proton and neutron.

Explanation

Proton (uud): two up quarks and one down quark.
Net charge =2(2e/3) – (e/3) = +e
Neutron (udd): one up quark and two down quarks.
Net charge (2e/3) + 2(-e/3) = 0