Fundamental particles of Atom:

Discovery of Electron:

Discovered by: Sir J.J. Thomson

Particles carrying negative charge were called negatrons by Thomson. The name negatron was changed to electron by Stoney.

Explanation:

Electron was discovered by utilizing faraday’s study of electrical discharge in partially evacuated tubes known as cathode ray tubes. Cathode ray tube is a long glass tube with two oppositely charged terminals. A gas is filled in it at low pressure.

Observations:

1. When the pressure inside the tube is 1 atmosphere and a high voltage of 10,000 volts is applied then nothing is observed inside the tube.

2. When pressure is reduced to 10^-2 atmosphere and high voltage is applied between the electrodes, a bright spot of light is formed on ZnS screen at the opposite end of cathode.This was caused by the rays which are coming from the cathode side. These rays are known as cathode rays.

Reason of generation of cathode rays:

At low pressure, there is enough space between the gaseous particles so that ions can accelerate to such an extent that when they strike another atom can eject electrons from it.

Electrons are the basic constituents of all the atoms because the characteristics of electrons do not depend upon the material of the electrodes.

Properties of Cathode rays:-

1. Cathode rays come out from cathode i.e. Perpendicularly from the surface of cathode. Their path does not depend upon the position of anode.

2. Cathode rays move in straight line & when an object is placed in the path of cathode rays, a sharp shadow is created.

3. Cathode rays produces heating effect when stopped by a thin metal foil. This is due to the fact that kinetic energy of the cathode rays is converted into heat energy when stopped by metal.

4. When a light paddle wheel is placed in the path of cathode rays, the wheel starts rotating. This indicates that cathode rays have kinetic energy.

5. Cathode rays can ionise the gas molecules through which they pass.

6. When an electrical field is applied in the direction perpendicular to the path of the cathode rays .They get deflected towards positively charged plate, indicating that rays are made up of negatively charged particles.

7. When a magnetic field is applied in the direction perpendicular to the direction of cathode rays, rays get deflected in the direction suggesting that they are consisting of negatively charged particles.

Determination of Specific Charge (e/m) of electron:

J.J. Thomson determines the e/m ratio of electron with the help of an experiment.

Principle:

Determination of charge by mass ratio of the electron is based on the fact that both electric & magnetic field deflect a beam of electrons. We apply electric and magnetic field in mutually perpendicular directions in such a way that a beam of electrons pass through them undeflected.

Knowing the values of V,E & B we can calculate e/m.

e/m = 1.75 X 10¹¹ CKg-¹

Determination of Charge of Electron:

R.A. Millikan determines the charge on an electron with the help of an oil drop experiment.

1. Measuring the terminal velocity of fall of a single charged oil drop acted on by gravity and viscous drag,

2. Applying Stokes’ law to determine the radius and mass of the drop, and

3. Measuring the upward terminal velocity in an opposing electric field.

The charge on the drop can then be calculated from these measurements.

Millikan repeated these measurements on thousand of drops. He found that the charge q calculated for each drop was some integral multiple of an elementary charge e. Thus

q=ne,   

Where e=1.60 10-19 C. This experiment provides a conclusive evidence of the quantisation of charge.

Discovery of Proton:

Discovered by: E. Goldstein

We know that atom is electrically neutral. When electrons were discovered, then Goldstein brought the idea that there must be some positively charged particles in the atom. This idea led to the discovery of proton. A ray of positively charged particles was discovered by Goldstein. These positive rays are known as canal rays. These rays were called positive rays by J.J. Thomson.

The smallest & lightest positive ion was obtained from hydrogen & was named as proton by Rutherford.

Characteristics of Anode rays:

1. Anode rays travel in straight line.

2. Anode rays possess positive charge therefore they move towards the negatively charged electrode.

3. The properties of anode rays depends upon the nature of gas taken in discharge tube.

4. The mass of the positive particles is same as the atomic mass of the gas inside the discharge tube.

Discovery of Neutron:

Discovered by: James Chadwick

He bombarded a thin sheet of beryllium by alpha particles, then neutral particles having a mass slightly greater than that of proton were emitted. He named these particles as neutron.