22/07/2026
CHAPTER 8 : JOURNEY INSIDE THE ATOM
(Grade 9)
Thomson's Discovery of Electron & Thomson's Atomic Model
IMPORTANT QUESTIONS AND ANSWERS
Key Fact: Discovery of radioactivity showed that atoms are not indivisible, as they emit smaller particles.
Questions & Answers
1. Q. What is radioactivity?
Ans. Radioactivity is the phenomenon in which certain elements emit invisible energy and particles called radiation.
2. Q. Who discovered the electron and in which year?
Ans. J. J. Thomson discovered the electron in the year 1897 while studying the conduction of electricity through gases at very low pressure.
Meet the Scientist:
J. J. Thomson — Discovered electron in 1897. Received the Nobel Prize in Physics in 1906 for his studies on electrical conductivity of gases.
3. Q. Describe Thomson's experiment to discover electron.
Ans. Thomson used a glass tube called cathode ray tube with a gas at very low pressure. He applied a high voltage across two electrodes — cathode (-) and anode (+). He observed rays moving from cathode to anode, called cathode rays.
4. Q. What are cathode rays?
Ans. Cathode rays are streams of negatively charged tiny particles emitted from the cathode in a cathode ray tube.
5. Q. List the properties of cathode rays.
Ans.
1.They travel in straight lines from cathode to anode.
2.They are negatively charged.
3.They have a very small mass (smaller than atoms).
4.Their nature is independent of the material of cathode or the gas filled in the tube.
5.They are a fundamental component of all atoms.
Charge of Electron:
The charge of an electron is -1.602 \times 10^{-19}and is taken as -1 by convention.
6. Q. What conclusions did Thomson draw from cathode ray experiment?
Ans.
Cathode rays are made of tiny, negatively charged particles called electrons.
Electrons are present in all atoms of every element.
7. Q. What is Thomson's model of an atom (Plum Pudding Model)?
Ans. Thomson proposed that an atom is a sphere of positive charge with electrons distributed throughout it. It is called the Plum Pudding Model, as electrons are like plums (seeds) embedded in a positively charged pudding.
8. Q. Write the limitations of Thomson's model.
Ans.
It could not explain how electrons are arranged in an atom.
It could not explain the results of the alpha particle scattering experiment later performed by Rutherford.
