Question

The S.I unit of radioactivity is
A) Becquerel
B) Curie
C) Rutherford
D) Roentgen

Answer 1

Hint: Radioactivity is the phenomenon of the spontaneous disintegration of unstable atomic nuclei to form atomic nuclei that are energetically stable.

Complete step by step answer:

Before knowing about the units of radioactivity, first we will learn about the definition of radioactivity. Radioactivity is nothing but the process in which unstable atomic nuclei disintegrate to form stable atomic nuclei with emission of electromagnetic radiation in the form of waves. The term radioactivity actually refers to the particles that are emitted from nuclei as a result of nuclear instability. Now let us learn about the unstable behaviour of radioactive elements. Generally, a nucleus comprises protons and neutrons. Protons are positively charged particles and neutrons are as the name suggests neutrally charged particles. We know that like charged particles repel each other. Hence, the protons repel each other. A strong force must exist between the protons and neutrons in a nucleus to overcome the electrostatic repulsion between protons. This strong force is called the strong nuclear force. This force is attractive over short distances but if the force was only attractive then it would pull the protons and neutrons together to constantly decrease the volume of the nucleus. But we know this is not true. This means the strong nuclear force must be repulsive at a very small range as well as being attractive over a slightly larger distance. This is the force that holds the nucleus together and the energy associated with this force is called the binding energy. The stability of a nucleus is determined by the amount of strong nuclear energy and the binding energy associated.Excess number of neutrons or protons cannot balance the binding energy and the strong nuclear forces making the nucleus unstable. An unstable nucleus tries to achieve a
stable state by releasing a neutron or proton and this is done via radioactive decay.
Basically, radioactivity is measured as the average number of radioactive decays(change in number of radioactive nuclei present) per unit time. It depends on the number of nuclei present as that instant. The radioactivity formula is given by
$$A\quad =\quad -\dfrac { dN }{ dt } $$ where, A denotes the radioactivity, N denotes the number of nuclei present.
The SI unit of radioactive decay is becquerel (Bq). It is named in honour of the scientist Henri Becquerel.
1 Bq = 1 disintegration per second. Hence, our answer is A. Becquerel.

Note: There is a chance for you to get confused by seeing the options because Curie is also a unit to measure radioactivity. But it is not an SI unit. Since, it is clearly mentioned in the question about the SI unit of radioactivity, the correct answer will only be Becquerel.