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We know that the protons and neutrons reside inside the nucleus. If the number of protons is equal to the number of neutrons, then the nucleus remains stable. If the number of protons is less than or more than the number of neutrons, then the nucleus is said to be unstable. Such unstable nuclei emit (release out excess neutrons or protons) certain radiations to attain a stable state. This was accidentally discovered by Henri Becquerel (1896), a French scientist. The radiations are called Becquerel rays.
The property of spontaneous disintegration of certain unstable atomic nuclei with the emission of certain radiations is called radioactivity.
The elements which are radioactive are called radioactive elements, e.g. radium, thorium, uranium, polonium etc. There are three primary types of radiation: alpha, a (fast moving helium atoms); beta, b (fast moving electrons); gamma γ (electromagnetic radiation). Some of the characteristics of a-, b- and g-rays are given in Table.
Characteristics of α-, β- and γ-rays
Property α β γ
Nature Positively charged helium nuclei (42 He) Negatively charged particles (same as that of electron) (0–1 e) Neutrally charged, i.e. they are high energy photons (no charge and no mass) (00 γ)
Velocity Description: 23217.pngth to Description: 23226.pngth the velocity of light 3% to 99% the velocity of light Same as that of light
Penetration power Very low (due to higher mass and lower velocity) 100 times that of a-particles (smaller mass and higher velocity) 100 times more than that of b-particles, i.e. 104 times more than that of a-particles


Property α β γ
Ionizing power High (due to greater kinetic energy) Description: 24133.pngth of a-particles (less kinetic energy) Very less (due to the absence of charged particles)
Effect on photographic plate Effect is moderate Effect is to a higher extent Effect is very less
Effect on zinc sulphide Causes fluorescence Causes fluorescence Causes fluorescence

Nuclear Stability and Radioactivity


The changing of one element to another by the disintegration of the nucleus by emitting radiation is called transmutation.

When a radioactive nucleus emits an a-particle, the remaining nucleus will be different from the original one because there is a loss of two protons and two neutrons.
22688 Ra Description: 23971.png 22286 Rn
When a radioactive nucleus emits a b-particle, there is no change in the mass number, but the atomic number of the newly formed nucleus increases by one unit.
146 C Description: 24372.png 147 N + 0-1 e

Half-life of a Radioactive Element

Half-life of a radioactive element is defined as the time taken for a radioactive sample of that element to get reduced to half its initial amount.
For example, half-life of radium-226 is about 1600 years. Suppose we start with 1 mg of radium, in 1600 years its radioactivity reduces to a half; in 3200 years, it reduces to a quarter; to one-eighth in 4800 years and so on.
Description: 22816.png

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