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INSIDE THE ATOM ; electron , proton and neutron

INSIDE THE ATOM  


John Dalton 1808, believed that matter is made up of extremely minute indivisible particles, called atom which can takes part in chemical reactions. These can neither be created nor be destroyed. However, modern researches have conclusively proved that atom is no longer an indivisible particle. Modern structure of atom is based on Rutherford’s scattering experiment on atoms and on the concepts of quantization of energy.

Composition of atom

It is now believed that the atom consists of several sub-atomic particles like electron, proton, neutron, positron, neutrino, meson etc. Out of these particles, the electron, proton and the neutron are called fundamental subatomic particles and others are non-fundamental particles.

Fundamental subatomic particles in an atom.

 Particle

Mass 

Charge 

Location 

Proton  

1 amu 

+1 

In the nucleus 

 Neutron 

1 amu 

 0

In the nucleus 

 Electron 

0.00055amu 

-1 

Outside the nucleus  

The atomic mass unit (amu) is 1/12 of the mass of an individual atom of 6c12 , i.e.  1.660× 10-27 kg 

Proton (p)

(1) Proton was discovered by Goldstein and is positively charged particle. It is a component particle of anode rays.
(2) Goldstein (1886) used perforated cathode in the discharge tube and repeated Thomson's experiment and observed the formation of  anode rays. These rays also termed as positive or canal rays. 
(3) Properties of anode rays
      (i) Anode rays are positively charged. 
      (ii) Anode rays travel in straight line.
      (iii)Anode rays are material particles.
      (iv) Anode rays may get deflected by external magnetic field.
      (v) Anode rays also affect the photographic plate. 
      (vi) The e/m ratio of these rays is smaller than that of electrons.             
      (vii) Unlike cathode rays, their e/m value is dependent upon the nature of the gas taken in the tube. It is maximum when gas present in the tube is hydrogen. 
      (viii) These rays produce flashes of light on ZnS screen.

       Other non fundamental particles 


Neutron(n)

(1)Neutron was discovered by James Chadwick (1932)) according to the following nuclear reaction,
(2) Neutron is an unstable particle. It decays as follows, 

Electron (e)

(1) It was discovered by J.J. Thomson (1897) and is negatively charged particle. Electron is a component particle of cathode rays.
(2) Cathode rays were discovered by William Crooke's & J.J. Thomson (1880) using a cylindrical hard glass tube fitted with two metallic electrodes. The tube has a side tube with a stop cock. This tube was known as discharge tube. They passed electricity (10,000V) through a discharge tube at very low pressure ( 10-2 to10-3mm Hg ). Blue rays were emerged from the cathode. These rays were termed as Cathode ray.
(3) Properties of Cathode rays 
 (i) Cathode rays travel in straight line.
 (ii) Cathode rays produce mechanical effect, as they can rotate the wheel placed in their path. 
(iii) Cathode rays consist of negatively charged particles known as electron.
 (iv) Cathode rays travel with high speed approaching that of light (ranging between 10-9 to 10-11 cm/sec)
 (v) Cathode rays can cause fluorescence. 




 
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