Question

Assertion (A): Nucleus of the atom does not contain electrons, yet it emits $\beta$-particles in the form of electrons.
Reason (R): In the nucleus, protons and neutrons exchange mesons frequently.
A. If both (A) and (R) are correct, and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct, but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.
E. If both (A) and (R) are incorrect.

Answer 1

Hint: Think about what force causes the radioactive decay of the nucleus to form ${\beta }^{-}$- particles i.e. electrons, and an electron antineutrino. Now, consider the force that enables the nucleons (protons and neutrons) to exchange mesons.

Complete answer:
The hadrons are a large family of particles that are made up of quarks and can take part in interactions of the strong nuclear force. These hadrons are divided into baryons and mesons. Mesons always have an even number of quarks, usually 2, a quark and an antiquark. Baryons consist of an odd number of quarks, usually three. Examples of baryons are protons (uud) and neutrons (udd).
Now, let us consider the reason in this question.
Reason (R): In the nucleus, protons and neutrons exchange mesons frequently.
Protons and neutrons are made up of mesons that are often exchanged and it is due to this that the nucleus is held together. As mesons are involved in the interactions of the strong nuclear force that keep nucleons bound together. Thus, the statement of reason is true and correct.
Now let us consider the assertion in this question.
Assertion (A): Nucleus of the atom does not contain electrons, yet it emits $\beta$-particles in the form of electrons.
This radioactive $\beta$-decay is caused due to the weak nuclear forces. For a neutron (udd) to change into a proton (uud), one of its down quarks has to undergo a flavour change and turn into an up quark while emitting an electron along with an electron antineutrino. This flavour change is only allowed by the weak nuclear interactions. Thus, the statement of assertion is true.
Now, consider the forces that are involved in both phenomena, $\beta$-decay requires the presence of weak nuclear forces and the exchange of mesons is a residual action of the strong nuclear forces. So, even though both the statements are true (R) is not the reason for (A) as they involve two different forces.

Hence, the answer is ‘B. If both (A) and (R) are correct, but (R) is not the correct explanation of (A).’

Note: Please do not get confused and mark ‘A. If both (A) and (R) are correct, and (R) is the correct explanation of (A).’ as your answer. Intuitively, it may seem that the degradation of the neutron into a proton and the change of a down quark into an up quark is caused due to some kind of instability during the exchange of the mesons, that is not the case. Strong nuclear interactions expressly prohibit flavour change and hence, exchange of mesons cannot be the reason for $\beta$-decay.