Question

Assertion: $\alpha$- rays have greater ionising power than $\beta$.
Reason: $\alpha$-particles carry +2 charge while $\beta$-particles carry only -1 charge.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Both Assertion and Reason are incorrect.

Answer 1

Understand the concept of radioactivity given in the chapter atoms and nuclei. Identify the difference between the two types of rays in terms of particles they emit, energy as well as the charge they carry. Based on this you can identify which ray has greater ionizing power.

Complete step by step solution:
Radioactive decay also is known as radioactive disintegration or nuclear disintegration is the process by which an unstable atomic nucleus loses energy in the form of radiation to gain stability. Any material containing unstable nuclei is considered radioactive.
Three of the most common types of decay are alpha decay, beta decay, and gamma decay, all of which involve emission of either protons or photons as to release energy.
-Alpha decay involves the release of an alpha particle from the nucleus.
-Beta-decay is said to occur when the atom either releases an electron and an antineutrino of a positron and a neutrino.
-In gamma decay, a radioactive nucleus first decays by the emission of either an alpha or a beta particle. The resultant product is still in an excited state and it decays to a lower energy state by emitting a gamma-ray or photon.
Alpha rays which propagate a beam of alpha particles are considered to be highly ionizing because of its doubly charged nucleus, large mass and relatively slow speed of propagation.
Therefore, we can conclude that $\alpha$- rays have greater ionising power than $\beta$. However, the reason for high ionizing power is not the charge carried by the respective particles.
The assertion statement, as well as the reason statement, are correct however the reason is not the correct explanation of the assertion.

Thus, the correct answer becomes option (B).

Note: The slow speed of propagation of alpha rays can cause multiple ionizations within a very small distance. So, we can clearly see that ionizing power is independent of the speed of propagation of a wave. It depends only on the ionizing character and features of the particles propagated through the wave.