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.

Question 1.31: 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.

Answer:

 Method - 1 

 Method - 2 

A proton has three quarks. Let there be n up quarks in a proton, each having a

charge of    

Charge due to n up quarks

Number of down quarks in a proton = 3 − n

Each down quark has a charge of 

Charge due to (3 − n) down quarks

Total charge on a proton = + e

Number of up quarks in a proton, n = 2

Number of down quarks in a proton = 3 − n = 3 − 2 = 1

Therefore, a proton can be represented as ‘uud’.

A neutron also has three quarks. Let there be n up quarks in a neutron, each having

a charge of	.
Charge on a neutron due to n up quarks
Number of down quarks is 3 − n,each having a charge of			.
Total charge on a neutron = 0

 Number of up quarks in a neutron, n = 1

Number of down quarks in a neutron = 3 − n = 2

Therefore, a neutron can be represented as ‘udd’.