Quick Revision

• 1870 → Crookes discovered cathode rays.
• 1879 → Crookes proposed negatively charged particles.
• 1895 → Roentgen discovered X-rays.
• 1897 → J. J. Thomson discovered electron and measured e/m ratio.

Cathode Rays and Electron Discovery – Key Points

1. Cathode ray particles travel at speeds of about 0.1–0.2 times the speed of light.
2. The accepted value of the charge-to-mass ratio (e/m) is:
   1.76×1011 C kg⁻¹
3. The value of e/m is independent of:
   • The cathode material used.
   • The gas present in the discharge tube.
4. This showed that cathode ray particles are universal in nature.
5. In 1887, it was observed that some metals emit negatively charged particles when exposed to ultraviolet light (photoelectric effect).
6. Certain metals also emit negatively charged particles when heated to high temperatures (thermionic emission).
7. The e/m value of these particles was found to be the same as that of cathode ray particles.
8. Therefore, all these particles were concluded to be identical in nature.
9. In 1897, J. J. Thomson named these particles electrons.
10. Thomson proposed that electrons are fundamental constituents of matter.
11. For his work on conduction of electricity through gases and discovery of the electron, Thomson received the Nobel Prize in Physics (1906).
12. In 1913, Robert A. Millikan performed the famous oil-drop experiment.
13. Millikan measured the charge of an electron as:
    e=1.602×10−19 C
    
14. He found that the charge on oil drops was always an integral multiple of the elementary charge.
15. This established that electric charge is quantized.
16. Using the values of e and e/m, the mass of an electron was calculated:
    m=9.11×10⁻³¹ kg

Important Years for NEET/JEE

• 1887 → Photoelectric emission observed.
• 1897 → J. J. Thomson discovered and named the electron.
• 1906 → Thomson received Nobel Prize.
• 1913 → Millikan’s oil-drop experiment.
• (e = 1.602 \times 10^{-19}) C
• (e/m = 1.76 \times 10^{11}) C kg⁻¹
• (m = 9.11 \times 10^{-31}) kg.