Atomic Structure:

Definition of Atom:-
Atom is the smallest particle of matter.
Atom consist on three fundamental particles which are electron proton and neutron.
Word atom has derived from the Greek language word "Atomos" which means indivisible or uncutable which mean it is indivisible.

Dalton's Atomic Theory:-
In 1808 Dalton gave an atomic theory which decsribes that atom is the smallest indivisible particle of matter which csn not be furthur sub divided in to smaller particles.
When electron, proton and neutron were discovered then the Dalton's theory was proved to be wrong.

Crooke's Discharge Tube Experiment:-

Experiment:-
Passage of electric current through gasses at low pressure discovery of Cathode Rays.
William crook was a worker at a German electric instruement maker Henrich G.Isler. He performed the experiment passage of electric current through gasses at low pressure.
For this experiment crook made a glass tube with a vacume pump to discharge gas from the tube. This glass tube is called Discharge tube or G.Isler tube.
Two electrodes of metal plate were fitted in the glass tube one cathode which is negative and other anode which is positive. Crook filled the tube with the gas at low pressure and connected cathode and anode with high voltage bettry. He found that no current passes through the gas, then he reduce the pressure in the tube by removing gas by the vacum pump.
He found that at 1mm or Torr current passes through the gas and the gas emitted light. When pressure was furthur reduced upto 0.01mm then emission of light stopped but some yellowish rays start emitting through the cathode and move toward anode. These rays are called cathode rays.

Cathode rays:-
Rays which emit through cathode in Crooke's discharge tube when the current is passes through the gas at very low pressure that is 0.01mm are called cathode rays.

Properties of cathode rays and discovery of electron (Cathode rays led to the discovery of Electron):-
Cathode rays have following properties.
• Cathode rays travel in staright line from cathode to anode
• cathode rays heat up the matter kn which they fall, thus they have energy.
• cathode rays make shadow of opaque rays and rotate the frictionless pin placed in their path. This shows that they are material in nature means made of particles.
• In 1897 J.J Thomson found that cathode rays are deflected toward positive plate when passes through an electric field. This shows that they contain negative charge.
• J.J Thomson measured the charge to mass ratio (e/m) of the cathode rays and found that this charge to mass ratio (e/m) does not depend on the nature of gas and the material of cathode.
• J.J Thomson found that cathode rays contain negatively charged particles.
• Stoney named the negative particles of cathode rays as Electron.

Conclusion:-
Electron are the fundamental particle of every matter.
Gold Stein's Experiment (Discovery of Proton From the Canal rays or Positive rays).
In 1886 Gold Stein used a perforated cathode and passes curreny through the gas at 0.01mm pressure. He found that some rays pass through the perforated of cathode. These rays are called canal rays or positive rays or Gold Stein rays.

Nature of Canal rays:-
Canal rays are the positive charged atoms of gas used in the tube and are formed when cathode rays strike with the atoms of gas in the tube then eject electrons of the gas atoms.

Properties of Canal Rays OR Positive Rays:-
Canal rays or positive rays have following properties.

• Canal rays travel in straight line toward cathode and pass through the perforates of cathode.
• canal rays are also material in nature means made of particles.
• They are deflected in electric field opposite to the cathode rays i-e towards negative plate, this shows that they contain positive charge.
• charge to mass ratio (e/m) of canal rays or positive rays depend on the nature of gas used in the discharge tube. If the atomic mass of gas atoms in the tube is increased then the (e/m) ratio of the rays decreases.
• The lightest positive particle with the highest e/m ratio was found from Hydrogen.  It was found that this particle has mass 1836 times more than the mass of an electron. This particle was named as Proton.
In this way proton was discovered from canal rays.

Conclusion:-
Proton are the fundamental particles of an atom which form mass of atom.

Chadwick's Experiment (Discovery of Neutron):-

Experiment:-
- Bombardment of alpha particles or Helium nuclei on the Beryllium nuclei
Explanation:-
Chadwick bombarded alpha particle or helium nuclei on the Beryllium nuclei and found that Beryllium nuclei changed to carbon nuclei with the emission of a particle having no charge and mass equal to the mass of proton. This particle was named as Neutron. Thus the neutron was discovered by the distingration of Beryllium nuclei by bombardment of Helium nuclei.

Plank's Quantum Theory:-
(Gave by Max Plank in 1900).
Max Plank gave the theory in 1900 about the absorption and emission or energy by elements. Main points of this theory are given below:
1) When an element is heated or cooled then the atom does not absorb or emit the energy continously, but in steps. Thus the absorption and emission of energy by atoms is discontinous process.
2) Atom absorb or emit energy in the form of quantum (small packet of energy).