Define the activity of a given radioactive substance. Write its S.I units?

Hint: A radioactive substance is a substance that contains an unstable nuclei. Radioactive decay is a process in which an unstable nucleus loses energy by the process of radiation. Radioactivity is a random process. The type of product produced after radioactive emission depends on the type of radioactive decay.

Complete step by step answer:
A radioactive substance is a substance that contains an unstable nuclei. Radioactive decay is a process in which an unstable nucleus loses energy by the process of radiation. Radioactivity is a random process. The type of product produced after radioactive emission depends on the type of radioactive decay.
Step-By-Step Explanation:

In the process of radioactivity, the unstable nucleus tries to attain a stable state by emitting alpha, beta or gamma particles. Also energy is released during this process. This process is known as radioactivity or radioactive decay.
Due to the very small size of the nucleus, it is impossible to predict the radioactive decay. Therefore, the radioactive decay is independent of the conditions like force, temperature and pressure. It just depends on the number of atoms around the nucleus.
Activity of a radioactive substance is also known as the rate of its radioactive decay.
If ‘A’ is the number of decays per unit time and ‘N’ is the total number of particles in a radioactive substance, then the rate of radioactive decay is written as
\[A = - \dfrac{{dN}}{{dt}}\] (number of disintegrations per second)
The S.I unit of activity is Becquerel (Bq.)
Negative sign indicates the time increase the number of undecayed nuclei present decreases.

Note:
In radioactive substances the atoms decay naturally. They give various types of radiation such as alpha particle radiation, beta particle radiation, gamma radiation etc. these radiations are very dangerous due to unstable nuclei atoms. Instability is due to the strong nuclear force that holds the nucleus of the atom together is not balanced by the electric force.