Discovery of electrons- study of cathode rays ,CLASS-11

Structure of atom

STRUCTURE OF ATOM

TOPICS TO COVER :-

1. The discovery of electron, proton and neutron and their characteristics
2. Atomic models (Thomson, Rutherford, and Bohr atomic models)
3. Quantum mechanical model of atom
4. Electromagnetic radiation and Plank's quantum theory
5. Photoelectric effect
6. Atomic spectra
7. de Broglie relation
8. Heisenberg uncertainty principle
9. Quantum numbers
10. Aufbau principle , Pauli exclusion principle, Hund's rule of maximum multiplicity
11. Electronic configuration of atoms and ions

1.Discovery of fundamental particles

Discovery of electrons - study of cathode rays

 


When a high voltage of about 10,000 volts is applied between the electrodes of a discharge tube ( Crookes tubes ) the following results are observed at different pressures:-




(i) When the gas pressure inside is 1 atm, no current flows between the electrodes. This is because gases are poor conductor of electricity.




(ii) When the pressure is reduced to about 10^-2 atm (about 1mm to 10 mm), the current starts flowing between the electrodes and a coloured glow is observed , the colour depending upon the nature of the gas taken.




(iii) When the pressure is further reduced to about 10^-4 atm (about 0.01 mm), the glow between the electrodes disappears but the current continues to flow and if a perforated anode is used , a faint greenish glow is observed on the glass wall behind the anode. This shows that some invisible rays are emitted from the cathode which passed through the holes of the anode and strike the glass wall (which may be coated with a phosphorescent material like Zinc Sulphide ZnS ). These rays were called cathode rays.

Properties of cathode rays




(i) They produce a sharp shadow of the solid object placed in their path. This shows cathode rays travel in straight lines.




(ii)If a light paddle wheel (e.g., that of mica) mounted on an axle is placed in their path, the wheel begins to rotate . This shows that cathode rays are made up of material particles.




(iii) When an electric field is applied on the cathod rays , they are deflected towards the positive plate of the electric field . This shows that cathode rays carry negative charge.
Similarly, when a magnetic field is applied on the cathode rays , these rays are deflected in a direction which shows that they carry negative charge.




(iv) When cathode rays strike a metal foil, the latter becomes hot. This indicates that cathode rays produce heating effect.




(v) They cause ionization of the gas through which they pass.
(vi) They produce X - rays when they strike against the surface of hard metals like tungsten(W) , molybdenum (Mo) etc.




(vii)They produce green fluorescence on the glass walls of the discharge tube as well as on certain other phosphorescent or fluorescent substances such as zinc sulphide (ZnS).




(viii) They affect the photographic plates.




(ix) They possess penetrating effect i.e. they can easily pass through thin foils of metals.They are stopped only by the metal foils of greater thickness.


Out of these properties two most important results are-
(i) Cathode rays are made up of material particles.
(ii) Cathode rays carry negative charge.
" The negatively charged material particles constituting the cathode rays are called electrons."

Thomson's Experiment for Determination of Charge/Mass(e/m) of the Electrons

R.A. MILLIKAN'S OIL DROP EXPERIMENT TO DETERMINE CHARGE OF AN ELECTRON

J.J. Thomson(1897) found charge/mass value to be -
e/m = 1.76 × 10 ⁸ C/g
R.A. Millikan (1917), with the help of oil drop experiment, found the nearest value of charge of an electron to be
e = 1.602 × 10^-19 C
Thus, the mass of an electron may be calculated from the values of e/m and e as follows:
mass = m = e/(e/m) = (1.602 × 10^-19 C)/(1.76 × 10 ⁸ C/g) = 9.11 × 10^-28g

ORIGIN OF CATHODE RAYS :-

(i)From the material of the cathode
(ii)From the gas inside the discharge tube due to bombardment of the gas molecules by the high speed electrons emitted first from the cathod.

Electron is the universal constituent of matter

In the discharge tube experiments, it is found that irrespective of nature of the gas taken inside the tube and the material of the electrodes , the ratio of charge/mass of the particles constituting the cathode rays (electrons) is the same.This proves that electron is the universal constituent of matter.