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1.What is meant by thermionic emission?

Emission of hot free electrons from a metal surface is called thermionic emission.  The minimum energy with which the electrons emitted is called threshold energy and these electrons are called photo electrons.   As these free electrons emit due to heating effect of metal they are also called thermions.If the temperature of the metal is higher.  Greater number of electrons are emitted. 

2.What is meant by radioactivity and radio-active disintegration?

Spontaneous emission of radioactive particles from the nuclei of heavy atoms is called radioactivity.  The radiations  namely alpha, beta and gamma radiations are called radioactive radiations and substances emitting such radiations are called radioactive substances. Due to continuous emission of these particles,mass of radio-active substance decays and this phenomenon is called radio-active integration.

3.What are alpha(α) rays?

Alpha radiation is nothing but the nucleus of helium.  Since it has two positive (+2 e ) charges its ionizing power is maximum.  alpha particles deflects in electric and magnetic fields.  It causes the effect of fluorescence.  Being doubly charged it causes maximum ionization in the gas through which it passes. 

4.What are beta(β) rays?

Beta particles are nothing but fast moving electrons.  The speed of beta particle is nearly 0.9 times the speed of light in vacuum.  These rays can produce the effect of fluorescence and affect the photographic plate. Beta rays can easily pass through the skin and cause more damage to human body. 

5.What are gamma(γ) rays?

Gamma rays are electromagnetic waves of wavelength nearly 10-3 Å.  Their wavelengths are those of very short X - rays, from which they differ only because they arise in atomic nuclei.  Whereas X -rays come from energy changes in the electrons outside the nucleus.  The ionizing power of gamma  radiations is least, nearly 1/100 of that of beta radiations.  The penetrating power of gamma rays is nearly 100 times that of beta particles.  They cannot be deflected in electric and magnetic fields as they have no charge.  Uncontrolled exposure of these radiations causes maximum damage to human body.  Controlled exposure of gamma radiations are used in the treatment of cancer. 

6.What are the dangers of radioactivity?

All three types alpha, beta and gamma radiations cause harmful biological effects on us.  These harmful effects can be short term effects, long term effects and genetic effects.  The short term and long term effects are bad for individual human beings  but  the long term effects affect the generation to come.  Only a small accidental exposure of these radiations is sufficient to cause these harmful effects.  Therefore some safety precautions are required to be taken. 

7. What are the necessary safety precautions against radioactive radiations?

Only a small accidental exposure of these radiations is sufficient to cause harmful effects.  Therefore for safety of persons handling these radioactive materials,some safety precautions are required to be taken they are as follows:
(i) The persons while handling radioactive substances will always put on lead lined aprons and lead  gloves.
(ii) The weak sources used at school should always be lifted with forceps, never held near the eyes and should be kept in their boxes when not in use.
(iii) A persons working at such place is always required to wear special film badge.  This badge indicates the amount of radioactive exposure on that person. No sooner this exposure     is beyond safety limits the person should be withdrawn from such places.
(iv) All radioactive substances should be handled with great care and always be put in thick lead containers.  Only a narrow opening should be allowed in these containers to emit these radiations.

8. What are Isotopes? 

Isotopes of an element are atoms which have the same number of protons but different numbers of neutrons. Isotopes have identical chemical properties since they have the same number of electrons and occupy the same place in the periodic table. 
Example. Hydrogen has three isotopes, with 1 proton, deuterium 1 proton and 1 neutron and tritium  with 1 Proton and 2 neutrons.  Ordinary hydrogen contains 99.99 per cent of   atoms.  Water made from deuterium is called 'heavy water'  (D2O) it has density of 1.108 g / cm3, it freezes at 3.8oC and  boils at 101.4oC.

9. What is meant by radioactive decay?

The emission of an α (alpha)or a  β( beta) particle from an unstable nucleus produces an atom of a different element, which may itself be unstable.  After a series of change a stable end element is formed. This is called radioactive decay. 

10.Explain alpha decay.

An alpha particle is a helium nucleus having 2 protons and 2 neutrons and when an atom decays by α emission, its nucleon number decreases by 4 and its proton number by 2.
Example.  When radium of nucleon number 226 and proton number 88 emits an α particle, it decays to radon of nucleon number 222 and proton number  86.  we can write.
                             
The values of atomic number and mass number must balance on both sides of the equation. 
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