Vidaara.orgClass 9 · Chemistry
CodeVID-C9-04-T1-01
Assignment — Discovery of Subatomic Particles & Early Atomic Models
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The proton was discovered through the study of:
- A.cathode rays
- B.canal rays
- C.X-rays
- D.gamma rays
2.
The charge on an electron is:
- A.+1
- B.0
- C.-1
- D.+2
3.
The neutron was discovered by:
- A.Thomson
- B.Goldstein
- C.Chadwick
- D.Dalton
4.
In Thomson's model, the atom is compared to a:
- A.solar system
- B.plum pudding / watermelon
- C.ladder
- D.spiral
5.
Protons and neutrons are together called:
- A.isotopes
- B.nucleons
- C.ions
- D.shells
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
State the charge and approximate mass of a proton and an electron.
7.
Describe Thomson's model of the atom.
8.
Why is an atom electrically neutral?
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
State the three observations of Rutherford's alpha-scattering experiment.
10.
List the conclusions Rutherford drew from his experiment.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Explain Rutherford's nuclear model of the atom and state its main limitation.
Answer Key
Section A — Multiple Choice Questions
- (B) canal rays
- (C) -1
- (C) Chadwick
- (B) plum pudding / watermelon
- (B) nucleons
Section B — Short Answer (2 marks)
- Proton: charge +1, mass ≈ 1 u. Electron: charge −1, mass ≈ 1/1840 u (taken as nearly 0).
- The atom is a positively charged sphere with electrons embedded in it like seeds in a watermelon; the positive and negative charges are equal, so the atom is neutral.
- It contains equal numbers of protons (+1 each) and electrons (−1 each), so the total positive and negative charges cancel out.
Section C — Short Answer (3 marks)
- Most α-particles passed straight through; a few were deflected by small angles; a very few (about 1 in 12,000) bounced almost straight back.
- Most of the atom is empty space; the positive charge and nearly all the mass are concentrated in a tiny central nucleus; electrons revolve around the nucleus.
Section D — Long Answer (5 marks)
- Rutherford proposed that an atom has a very small, dense, positively charged nucleus at its centre that holds nearly all the mass, while the electrons revolve around the nucleus in circular paths, and most of the atom is empty space. Limitation: a revolving electron is an accelerating charge, and an accelerating charge must continuously radiate energy; losing energy, the electron would spiral inward and fall into the nucleus, so the atom would be unstable. Since atoms are actually stable, Rutherford's model could not explain the stability of the atom.
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