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.

Question

Philoid · Accepted Answer

Let there be n up quarks.

Number of down quarks = 3-n (Since there are 3 quarks in a proton)

Charge on n up quarks = where e is the charge of a proton

Charge on 3-n down quarks =

In case of a proton:

Since the charge on a proton is e,

⇒

⇒ n = 2

So, 3-n = 3-2 = 1

Hence, there can be two up quarks ⇒, 2u and one down quark ⇒, 1d in a proton.

In case of a neutron:

Since a neutron is electrically neutral,

⇒

⇒ n = 1

So, 3-n = 3-1 = 2

Hence, there can be one up quark i.e. 1u and two down quarks i.e. 2d in a neutron.