Introduction
Henri Bacqural discovered that Uranium atoms (z = 92) emit highly penetrating radiations that could penetrate paper, glass and even aluminium. On the basis of his experimental results, he explained the phenomenon of radiation.
Definition
The phenomenon of spontaneous disintegration of nucleus of atoms is known as radioactivity.
Explanation
Radioactivity is a self-disrupting activity exhibited by some naturally occurring elements. It has been found, that the elements with atomic number greater than 83 are unstable and emit certain type of radiations. Such substances (e.g. Uranium, Radium, Thorium) are called Radio-active substances and the radiations emitted from their nuclei are called radio active radiations and the phenomenon is known as Radioactivity. Rutherford and his co-workers proved that the radiations emitted by a radio active substance are of three different types.
Experiment
Radio Active radiations can be separated by applying electric or magnetic field to the element. A small amount of radioactive substance is placed at the bottom of a cavity drilled in a block of lead. When the narrow beam of radioactive rays is allowed to pass through the space between the two charged plates, the path of some rays bend. A similar effect is observed in the presence of magnetic field.
Results Obtained
The conclusion that were made fro the experiment are
1. α – Particles
The rays towards the negative plate indicate that they consist of positively charged particles. These were named as α-rays.
2. β – Particles
The rays bending towards the positive plate indicate that they consist of negatively charged particles. These were named as β (beta) rays.
3. γ – Rays
The rays that go undeflected indicate no charge and are therefore energetic photons or γ (gamma) rays.
Properties of α – Particles
1. α – Particles are Helium nuclei. The charge of a α-particle is twice the charge of a proton and its mass is four times than that of a-proton.
2. The speed of α-particles is 1/100 times the speed of light.
3. They produce fluorescence and effect the photographic plate.
4. α – Particles have low penetrating power.
5. They have high ionization power.
6. When a nucleus zXA disintegrates by the emission of an α-particles, its charge number (z) decreases by 2 and mass number (A) decreases by 4.
zXA —-> Z2 (VA.4) + α – Particle
Properties of β – Particles
1. β – Particles are electrons with more energy as compared to ordinary electrons because their origin is nucleus and not the atomic orbits.
2. The speed of β – particles is 1/10 times the speed of light.
3. They produce fluorescence and affect the photographic plate.
4. β – particles have greater penetrating power then α-particles.
5. They have low ionizing power.
6. When a nucleus zXA disintegrates by the emission of β – particle, its charge number (Z) decreases or increases by 1, while mass number remains same.
zXA —-> z+1 γA + -1βº (electron)
zXA —-> z-1 γA + +1βº (positron)
Properties of γ – Rays
1. γ – Rays are energetic photons and have no charge. They are similar to X – rays but more energetic.
2. They travel with the speed of light.
3. The produce fluorescence and affect the photographic plate.
4. Their penetrating power is very high.
5. They do not have any ionization power.
6. When γ – Rays emit out from the nucleus of a radio active substance, then the mass number (A) and charge number (Z) remain same
zXA —-> zXA + γ – Rays
Where zXA represents the nucleus in excited state.
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