Define the activity of radioactive substances. Write its S.I. Units.

Hint: Radioactive substances are unstable, and the dangerous substances that produce radiation. This substance is unstable because the stronger nucleus force that holds the nucleus of the atom together is not in balance with the electric force that wants to push atoms apart, and these make radioactive substances to decay.

Complete step by step solution:
Radioactivity decay is the process by which an unstable atomic nucleus loses energy by radiation. Three of the most common decayes are alpha, beta and gamma decay.
The activity of radioactive substances is defined as the rate of a number of the disintegration of the given sample per second or number of unstable atomic decay per second in the given sample. This activity is determined by counting the number of particles and pulses of electromagnetic energy by radiation detectors and electronic circuits.

Its SI unit is Becquerel (Bq). where 1 Bq is the one disintegration per second.
The old standard unit was Curie which is equal to\[ = 3.7 \times {10^{10}}Bq\].

Note: It is to be noted here that the question is only asking for the SI units and not for any other units. There are different ways of indicating the units such as CGS units and MKS units but SI units are to be used unless and until the question demands any other units.