Thomsons Model and Its Limitations - UNSOLVED PRACTICE SET

Q7. Describe Thomson's plum pudding model of the atom with a diagram.

Q8. How did Thomson's model account for the electrical neutrality of the atom?

Q9. What was Thomson's estimate of the size of an atom compared to the size of the electron?

Q10. Why did Thomson assume that the positive charge was spread uniformly throughout the atom?

Q11. Name two predictions of Thomson's model that were contradicted by Rutherford's experiment.

Q12. How did Thomson determine the charge-to-mass ratio (e/m) of the electron?

Q13. Describe Thomson's model of the atom in detail. Explain how it accounted for the known properties of atoms and why it was considered a significant improvement over Dalton's model.

Q14. Discuss the limitations of Thomson's model. How did Rutherford's alpha scattering experiment specifically contradict the predictions of this model?

Q15. Explain J.J. Thomson's experiment to determine the charge-to-mass ratio of the electron. Describe the apparatus and the measurements made.

Q16. In Thomson's experiment to measure e/m, electrons are accelerated through a potential difference of 500 V and then subjected to electric and magnetic fields.

(a) Calculate the kinetic energy gained by the electrons in eV.

(b) Calculate the velocity of the electrons.

(c) If the electric field is 2 × 10⁴ V/m and the magnetic field is 2 × 10⁻³ T, and the fields are adjusted so that the electrons travel undeflected, calculate the e/m ratio.

(d) Using the known value of e = 1.6 × 10⁻¹⁹ C, calculate the mass of the electron.

Q17. According to Thomson's model, an atom of radius 10⁻¹⁰ m consists of a uniform sphere of positive charge with electrons embedded in it. For a hydrogen atom:

(a) Calculate the volume of the atom.

(b) If the positive charge is uniformly distributed, calculate the electric field at the centre of the atom.

(c) Explain why an alpha particle passing through such an atom would experience only a very small deflection.

Q18. In your school's physics exhibition, a student creates a model to demonstrate Thomson's plum pudding model.

(a) She uses a spherical bowl of diameter 20 cm to represent the atom. She places 10 small plastic balls (each representing an electron) randomly inside the bowl. The bowl is filled with a uniform mixture representing the positive charge. Explain how this model illustrates Thomson's concept.

(b) The student then calculates that if the actual atom has a diameter of about 10⁻¹⁰ m and the electron is about 10⁻¹⁵ m in size, what would be the equivalent size of her plastic balls if scaled proportionally? Discuss whether this scale model is practical.

(c) A classmate points out that in Thomson's model, electrons should be stationary, but we know they are moving. Explain why stationary electrons would be unstable according to classical electromagnetism, and how this was another limitation of Thomson's model.

(d) Thomson's model was popular in the early 1900s. Explain why scientists accepted it before Rutherford's experiment, and what experimental evidence would have been needed to disprove it earlier.