Thomsons Model Plum Pudding - UNSOLVED PRACTICE SET
Chapter: Structure of the Atom | Topic: Thomsons Model Plum Pudding
THOMSONS MODEL PLUM PUDDING - UNSOLVED PRACTICE SET
Topic: Thomsons Model Plum Pudding
Multiple Choice Questions
Q1. According to Thomson's model of the atom:
- The atom is a solid sphere with no internal structure
- The atom is a positively charged sphere with electrons embedded in it like seeds in a watermelon
- The atom has a dense central nucleus with electrons revolving around it
- The atom consists only of electrons
Q2. Thomson's model is also known as the:
- Nuclear model
- Planetary model
- Plum pudding model
- Quantum model
Q3. In Thomson's model, the positive charge is:
- Concentrated at the centre
- Spread uniformly throughout the atom
- Carried by electrons
- Present only on the surface
Q4. Thomson proposed his model to explain:
- The discovery of the nucleus
- The existence of electrons and the overall neutrality of the atom
- The existence of neutrons
- The line spectrum of hydrogen
Q5. The main limitation of Thomson's model was that it could not explain:
- The existence of electrons
- The results of Rutherford's alpha scattering experiment
- The neutrality of the atom
- The discovery of canal rays
Q6. Thomson compared his model of the atom to:
- A solar system
- A plum pudding or watermelon
- A solid ball
- A spinning top
Short Answer Questions
Q7. Describe Thomson's plum pudding model of the atom with a simple diagram. What are the two main components of the atom according to this model?
Q8. Why did Thomson propose that electrons are embedded in a positively charged sphere? What experimental evidence led him to this conclusion?
Q9. Your grandmother makes a traditional Indian sweet called "rasgulla" — soft white cheese balls in sugar syrup. Your teacher says Thomson's model is like a rasgulla with raisins embedded in it. Explain this analogy and identify what the rasgulla and the raisins represent.
Q10. What was the main achievement of Thomson's model? What was its biggest weakness that led to its replacement?
Q11. Compare Thomson's model with Dalton's atomic model. How did Thomson's model improve upon Dalton's ideas?
Q12. If Thomson's model were correct, what would happen to alpha particles fired at a thin gold foil? Explain your reasoning.
Long Answer Questions
Q13. Describe Thomson's plum pudding model in detail. Draw a labelled diagram showing the distribution of positive charge and electrons. Explain how this model accounted for: (i) the existence of electrons, (ii) the overall neutrality of the atom, and (iii) the stability of the atom. What were the two major limitations of this model?
Q14. A student draws Thomson's model as a circle with electrons randomly scattered outside the positive sphere. Another student draws electrons embedded inside the positive sphere. Which drawing is correct? Help both students by:
(a) Identifying the correct representation.
(b) Explaining why electrons must be inside the positive sphere.
(c) Describing what would happen if electrons were outside the positive sphere.
(d) Explaining why the model is called "plum pudding" and not "raisin bread."
Q15. "Thomson's model was a stepping stone, not a destination." Discuss this statement by explaining: (i) what Thomson's model got right, (ii) what it got wrong, (iii) how Rutherford's experiment disproved it, and (iv) why it is still important to study Thomson's model even though it was incorrect.
Numerical / Application-Based Problems
Q16. According to Thomson's model, an atom of gold (atomic number 79) contains 79 electrons embedded in a uniform positive sphere.
(a) If the total positive charge is +79e, calculate the total positive charge in coulombs. (e = 1.6 × 10⁻¹⁹ C)
(b) If the radius of a gold atom is approximately 1.44 × 10⁻¹⁰ m, calculate the volume of the atom assuming it is a sphere.
(c) If the positive charge is uniformly distributed, calculate the charge density (charge per unit volume).
(d) Thomson's model predicted that alpha particles would pass through atoms with only slight deflection. Calculate the expected deflection angle if an alpha particle with kinetic energy 5 MeV passes through this uniform positive charge distribution.
Q17. A classroom activity involves creating a model of Thomson's atom using a spherical balloon and small beads.
(a) If the balloon has a diameter of 20 cm and represents the positive sphere, what would be the scale if a real atom has a diameter of 2 × 10⁻¹⁰ m?
(b) If 10 beads represent electrons, and each bead has a mass of 0.5 g, calculate the total mass of the "electrons" in the model.
(c) If the balloon is filled with air at a pressure slightly above atmospheric pressure, what does the air pressure represent in Thomson's model?
(d) What limitation of Thomson's model can students discover by trying to make the beads stay evenly distributed inside the balloon?
Q18. Thomson's model was proposed before the discovery of the nucleus. Modern atomic theory tells us that the nucleus contains 99.9% of the atom's mass but occupies only 10⁻¹⁵ m of space, while the atom's size is about 10⁻¹⁰ m.
(a) Calculate the ratio of the size of the atom to the size of the nucleus.
(b) If the nucleus were the size of a cricket ball (7 cm diameter), what would be the size of the atom at the same scale? Express your answer in kilometres.
(c) Why does this enormous empty space in the atom contradict Thomson's model of a uniform positive sphere?
(d) How does this scale analogy help us understand why Rutherford was so surprised by his experimental results?