Structure Of Atom (JEE & NEET) || Chemistry || Class 11th || Short Notes || Formulas|| Video Lecture

Chemistry study of composition, structure, properties and change of matter.

 The existence of atom has been proposed since the time of early Indian and Greek philosopher who were on the view to that atoms are the fundamental building blocks of matter.

Atom Models

The Atomic Theory of Matter was first proposed by John Dalton in 1808.

 Dalton's Atomic Model :-

This atomic model is based on the laws of conservation of mass, definite proportion and multiple proportion. 

1. Elements are composed of minute indivisible particles called atom.
2. Element of same element are same in mass or size and element of different element are different in mass or size.
3. Chemical compounds are formed by the union of two or more atoms of different elements.
4. Atoms always combine in simple whole number ratios.
5. Chemical reaction only rearrange how atoms are combined in chemical compounds, atoms themselves don't change.

Cathode Ray Tube (CRT) Experiment:

In mid 1850s, many scientist mainly Faraday begin to study electrical discharge in partially evacuated tubes known as cathode ray discharge tubes. 

A cathode ray tube is made of glass containing two thin pieces of metal comma called electrodes, shield in its. 

The electrical discharge through the gas could be observed only at very low pressure and at very high voltage. The pressure of different gases could be adjusted by evacuation. When sufficiently High Voltage applied across the electrodes, current starts flowing through a system of particles moving in the tube from the negative electrode(Cathode) to the positive electrode(Anode) . These were called cathode rays or cathode ray particles.

The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays after passing through anode, strike the zinc sulphide coating, a bright spot on the coating is developed.

The results of these experiments are summarised below:-

1. The cathode rays start from cathode and move towards the anode. 
2. These rays themselves are not visible but their behaviour can be observed with the help of certain kind of material called fluorescent which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescent on the television screen coated with certain fluorescent materials. 
3. In the absence of electrical or magnetic field, these rays travel in straight lines.
4. In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons. 
5. The characteristic of cathode rays (electrons) do not depends upon the material of the electrode and the nature of gas present in the cathode ray tube.

Thus, we can conclude that electrons are basic constituent of all the atoms.

Charge To Mass Ratio Of Electron

In 1897, British physicist JJ Thomson measure the ratio of electric charge to the mass of electron by using cathode ray tube and applying electrical and magnetic field perpendicular to each other as well as to the path of electron.

Thomson tell that the amount of deviation of the particles from their path in the presence of electrical and magnetic field depends upon:-

• The magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is interaction with the electric or magnetic field and thus greater is the deflection. 
• The mass of the particle.
•  The strength of the electrical or magnetic field.

 Discovery Of Protons And Neutrons

Electrical discharge carried out in the modified cathode ray tube led to the discovery of particles carrying positive charge, also known as Canal rays. The characteristic of these positive charged particles are listed below:-

• Unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions.
• The charge to mass ratio of the particles is found to depend on the gas from which these originate.

•  Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
•  The behaviour of these particles in the magnetic or electric field is a positive that observed for electron or cathode rays.

The smallest and lightest positive Ion was obtained from hydrogen and was called proton. This positively charged particle was characterized in 1919.

Later, I need was felt for the presence of electrically neutral particle as one of the constituent of atom. These particles were discovered by Chadwick, by bombarding a thin sheet of beryllium by Alpha particle, when electrically neutral particles having a mass slightly greater than that of the proton was emitted. He named these particles as neutron.

Thomson Model Of Atom

One suggestion made by the British physicist JJ Thomson in 1898, was that atom are just positively charged lumps of matter with electrons embedded in them, like raisins in the fruitcake.

Read More.... 

Drawbacks

1. Considers mass to be evenly spread across the atom.
2. Its a static model, doesn't reflect the movement of electrons.
3. It couldn't explain the stability of an atom.

Rutherford's Nuclear Model Of Atom

Alpha particles are relatively heavy (almost 8000 electrons masses) and dose used in the experiment had high speed. So, it was clear that powerful forces were needed to cause such mark deflection.

Read More.... 

Observations:-

• About 92% Alpha particles passed undeflected.
• Some Alpha particles were scattered by small angles.
• A few word deflected by large angles.
• Almost one in 20,000 retract its path. 

Conclusion:-

• Most of the space in an atom is empty. 
• Positive charge and most of the mass is concentrated at a small point which was called nucleus.
• Volume of nucleus is negligible small compared to volume of an atom.
• Electron revolve around the nucleus due to the electrostatic force between positive nucleus and negative electrons.

Drawbacks:-

• It couldn't explain nuclear collape. 
• It couldn't explain line spectra for various element.
• It couldn't explain the energies and distribution of electrons around nucleus.