Structure of Atom
Right from beginning of human civilisation a question recently has been arising in the minds of contibutry philosophers, "What is the structure of matter". In this regard Kanad dating back 500 years proposed practically theory of matter. For example: Matter consists of small particles. These particles were given the name 'Atom'.
John Dalton a british chemist proposed atomic theory in the beginning of 19th centuary. His theory was supported by the laws of chemical combination. One of the important fact regarding his atomic theory was atoms are individuals. In later half of 19th centuary Dimitry Mandeve develop a periodic table in which elements were classified on the basis of similarity of their properties.
This led to a natural question, "Why the atoms of different elements show similarity in their chemical properties. There was natural answer too, "If the atoms of different elements exinite similarity in their properties there must be some sort of compositional similarity of atoms if different elements. This mystery was decoded by JJ Thomson and his when they proposed electron theory of matter says electron are universal constituents of matter.
Discovery of Proton
Glodstien a German scientist repeated the J J Thomson experiment by modifying the discharge tube. He used a Perforated Cathode and a gas at low pressure was placed inside the discharge tube. As shown in the figure, zinc screen placed behind the Perforated Cathode blinking lighted spots where observed when it was analysed by electric and magnetic field. The direction of deflection shown by the anode ray where found to be opposite to the shown by Cathode rays. This let to a conclusion that the so called anode rays where stream of invisible positivity charged particles.
The E/M of the anode rays where found to be different for different gases used in the discharged tube. The value of E/M was found to be maximum for hydrogen gas. Which is about 9.58 × 104 C/Gram. Goldstein study the values of E/M of different gases used his sample perception about the higher value of E/M of hydrogen gas used in the discharged tube. The mass of positively charged particles present in the anode rays where determined by mass spectrographic technique. Surprisingly the anode rays produced by hydrogen gas the mass of positive charged particles where almost equal to mass of hydrogen atom. This result led Goldstein to assume that there is existence of positively charged particles in the nucleus of hydrogen atom. This positively charged particle the name 'Proton' was given. This is how proton was discovered.
For Hydrogen gas used in the discharged tube.
After the discovery of proton, a new dilema came that was misusing mass of atom.
For example : If we take a hydrogen atom it consist of seven protons and seven electron which accounted for the mass of nitrogen atom to be seven times the mass of hydrogen atom which infect was the half of the atomic mass of nitrogen. This dilema of missing mass led the contemporary physicists to think about the existence of electrically neutral particles having the same mass as the proton has.
Discovery of Neutron
In 1932, James Chadwick discovered another subatomic particle having no charge with a mass nearly equals to that of proton. Neutrons are present in the nucleus of all atoms except hydrogen. Neutron represented by 'n'. And in the year 1932 the problem of missing particles gone.
[ Quick Fact : Atomic number ( Z ) = Number of protons = Number of Neutrons ]
Mass number ( A ) = Atomic no. ( Z ) + No. of Neutrons
= No. of protons + No. of Neutrons
= No. of electrons + No. of Neutrons
Or we can also express this in this way :
Number of Neutrons = Mass no. ( A ) - Atomic no. ( Z )
= Mass no. ( A ) - No. of Protons ( p )
= Mass no. ( A ) - No. of electrons ( e )
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