What is an atom?
‘New System of Chemical Philosophy’ is a book about atom written by John Dalton in 1703.
Dalton Atomic Theory
According to this theory atom cannot be divided, but after some years it is proved that atom can be divided further by many scientists like J.J.Thomson, L.Rutherford, N.Bohr, Chadwick etc.
Constituting particles of atom
Discovery of Electron
Air is bad conductor of electricity so vacuum pump is connected to reduce pressure to 0.02mm inside discharge tube currents starts flowing between electrodes and light is emitted. On further reducing pressure in discharge tube greenish yellow color fluorescence occur. As these rays emerging from cathode, Sir J.J.Thomson named them as cathode rays.
Deflection of cathode rays towards positively charged plate in electric field proves that these rays carry negatively charged particles.
These negatively charged particles are named as electrons.
Properties of Cathode Rays
|Production of Cathode Rays|
2. Velocity of cathode rays and velocity of light are approximately equal.
3. On applying electric field in the path of cathode rays, cathode rays turn towards +vely charged plate that proves cathode rays are made up from negatively charged particles.
4. Cathode rays rotate light wheel placed in their path that proves cathode rays are made from particles having mass.
5. Cathode rays pass through thin metal foil and it gets slightly heated up by action.
6. These rays produce fluorescence at walls of glass tube.
7. Cathode rays ionize gases and also affect photographic plate.
8. When these rays strike any metal with high melting point (like tungsten W) they produces X-Rays.
What is X-rays?
X-rays are electromagnetic radiation , X-rays was discovered by W.K.Roentgen in 1895. X-rays are also known as Roentgen rays.
Why X-rays are used in medical sciences?
X-rays are used in medical sciences because x-rays have high penetrating power.
Determination of charge to mass ratio of electron (e/m)
J.J.Thomson conduct many experiments that charge to mass ratio of an electron remains same, irrespective of nature of gas and nature of cathode electrode material.
value of e/m = -1.76×108 coulombs/gram
Millikan’s Oil Drop Experiment
Millikan’s Oil Drop Experiment or Determination of charge of electron is conduct by an American Scientist R.A.Millikan, who perform an experiment on the charge on oil drops. R.A.Millikan perform several experiments to calculate charge on oil drops and he gets every time its value equal to -1.6×10-19 coulomb.When these results associated with results of cathode rays then conclude that charge present on particle of cathode rays is -1.6×10-19 coulomb.
Calculation of mass of electron
As we know e/m = -1.76×108 coulombs/gram
e = -1.6×10-19 coulomb
(e/m)/e = (-1.76×108)/(-1.6×10-19)
m = 9.102×10-28 gram
m = 9.102×10-31 kilogram
Mass of electron in comparison with atom
Mass of electron in comparison with atom is described below-
Mass of 1 mole of Hydrogen = 1.008gms
Number of hydrogen atom in 1 mole = 6.023×1023
Mass of 1 atom of hydrogen = 1.008/6.023×1023
Mass of electron is 9.109×10-31
= Mass of 1 atom of hydrogen/Mass of electron
= (1.67×10-27)/(9.109×10-31) = 1837
Mass of an electron is 1/1837 th the mass of a hydrogen atom.
Discovery of Proton
As we know electron is negatively (-vely) charged particle but atom is electrically neutral so there should be some particles which have positive (+ve) charge to neutralize negative (-ve) charge electron.
In 1886, a German scientist E.Goldstein established the presence of +vely charged particles. These positively charged rays travel from anode to cathode so called as anode rays or positive rays.
Why anode rays are called as canal rays?
Anode rays passes through canals or perforation in cathode so called as canal rays.
Properties of Anode Rays
1. Anode rays always travel in straight line.
2. Anode rays rotate light wheel placed in their path that proves anode rays are made from particles having mass.Which produces mechanical action.
3. On applying electric field in the path of anode rays, anode rays turn towards -vely charged plate that proves that anode rays are made up from positively charged particles.
|Production of Anode Rays|
Determination of charge to mass ratio of proton (e/m)
On the basis of many experiments performed for anode rays, scientist Wein concluded that charge to mass ratio of proton changes with change in nature of gases present in discharge tube. This value (e/m) is maximum for hydrogen gas = 9.58×104 coulomb per gram. “Positively charged particle of hydrogen is fundamental particle of matter that is called proton.”
Charge on proton
Charge on proton is equal to charge on electron but is of opposite nature.
Charge on proton = 1.602×10-19 coulomb.
Mass of the proton
For hydrogen gas,
e/m = 9.58×104 coulomb per gram
charge of electron = 1.602×10-19 coulomb
m = (e)/(e/m) = (1.602×10-19)/(9.58×104)
m = 1.67×10-24 gram
m = 1.67×10-27 kg
mass of proton is 1837 times more than mass of electron and is equal to mass of an hydrogen atom.
Discovery of Neutron
Mass of atom is more than the mass of total proton and electron present in atom, which suggest the presence of another particle in atom which lead the discovery of neutron.
Chadwick in 1932 discover neutral particles of mass equal to mass of proton by bombarding beryllium metal with stream of fast moving particles through cyclotron. These particle are neutral in nature so named as neutron.
Mass of neutron = 1.6748×10-27 kg
There is three fundamental particles in atom
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