Question

Why are neutrons heavier than protons?

Answer 1

This question is based on the subatomic particles present in the atom. The particles that are found in the structure of the atoms and are smaller in size than the atom are known as subatomic particles. There are two types of subatomic particles, elementary and composite. Neutron is the heaviest known subatomic particle.

Complete answer:
The nucleons is the combined term used for both protons and neutrons. We will study about a new fundamental particle ‘Quark’. Quarks are the smallest indivisible fundamental particle known till date. There are primarily two types of quarks we’ll study from the six types of quarks known. They are the ‘up’ quark and the ‘down’ quark. The proton and the neutron differ in their compositions by means of quarks. A nucleon contains at least 1 up quark and one down quark.
Remember that the charge on the up quark is $+ \dfrac{2}{3}$ and that of the down quark is $- \dfrac{1}{3}$ . The proton contains two up and 1 down quark, whereas the neutron has 2 down and 1 up quark. The charges can be given as:
${Q_p} = {Q_u} + {Q_u} + {Q_d} = + 1$
${Q_n} = {Q_d} + {Q_d} + {Q_u} = 0$
These values justify the charges on the proton and neutron. It was found that the mass of the down quark is more than that of the up quark. Since the neutron has 2 down quarks, the mass of the neutron was found to be slightly greater than proton. Because of this the neutron decays and protons don't.

Note:
If the down quark was lighter than the up quark, then the proton would easily decay into a neutron. The whole universe would be composed of only hydrogen atoms then, filled with neutrons and neutrinos. Also, if the down quark was very heavier, the neutron would decay very quickly.