Question

A radioactive substance emits:
A. $\alpha ,\beta$ and $\gamma$ simultaneously
B. $\alpha$ or $\beta$
C. X-rays and $\gamma$ rays.
D. None of these.

Answer 1

Due to nuclear instability, nucleus emits particles. This is called radioactivity. The nucleus has a conflict between two forces. There are many elements which possess radioactivity. The instability of the nucleus is due to the unbalanced forces.

Complete step by step answer:
The smallest part of an element is known as an atom. It has a nucleus at the center. Every atom consists of electrons and protons at the nucleus. The protons and neutrons together are known as nucleons. There are two types of forces that occur in the nucleus. They are:
- Coulomb repulsive forces-Forces between protons and
- Nuclear forces-Forces between nucleons.
The nuclear force is an attractive force. Since it is a very strong force, it negates the effect of the force between protons and binds the nucleus together. When the nucleus is excess in internal energy, the nucleus becomes stable.
Radioactivity is something related to an unstable nucleus. A stable nucleus does not emit radiation. The nuclei of naturally occurring elements like Uranium, Thorium, Radium and Polonium are unstable and emit invisible rays which are called radiations. These are not emitted simultaneously. This property of emitting rays is called radioactivity. The heavy elements that emit rays are called radioactive elements. Thus radioactivity can be defined as the phenomenon in which the nucleus of an element undergoes spontaneous disintegration with emission of certain types of radiations. It emits $\alpha$or $\beta$ rays.

Hence the option B is correct.

Additional information:
Radioactive substances emit particles and energy due to the breaking down of the nuclei of unstable atoms. There are three types of radiations called $\alpha ,\beta$and $\gamma$ rays.
Some important features of $\alpha$, $\beta$ and $\gamma$ rays are given in the table below:

$\alpha$ rays	$\beta$ rays	$\gamma$ rays
Positive charged	Negative charged	No charge
High ionization energy	Low ionization energy	Low ionization energy
Low penetrating power	High penetrating power	High penetrating power

Note:
The instability of the nucleus may lead to an excess of either protons or neutrons. It tends to achieve stability by ejecting nucleons or energy in other forms. A nucleus is unstable when the forces among the particles are unbalanced.