Vidaara.orgClass 9 · Chemistry
CodeC9-CHEM-4-CW
Structure of the Atom — Chapter Worksheet
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Show all working for the numerical and long-answer questions.
Section A (1 mark each)
6 × 1 = 6 marks
1.
Who discovered the neutron?
- A.Thomson
- B.Goldstein
- C.Chadwick
- D.Bohr
2.
The charge on an electron is:
- A.+1
- B.0
- C.-1
- D.+2
3.
The maximum number of electrons in the M shell (2n²) is:
- A.2
- B.8
- C.18
- D.32
4.
Atoms with the same mass number but different atomic numbers are:
- A.isotopes
- B.isobars
- C.ions
- D.nucleons
5.
The valency of magnesium (2, 8, 2) is:
- A.0
- B.2
- C.6
- D.8
6.
The number of neutrons in 2311Na is:
- A.11
- B.12
- C.23
- D.34
Section B (2 marks each)
4 × 2 = 8 marks
7.
State the charge and approximate mass of the electron, proton and neutron.
8.
Write the electronic configurations of carbon (Z = 6) and argon (Z = 18).
9.
Define isotopes and give one example.
10.
Why is an atom electrically neutral?
Section C (3 marks each)
2 × 3 = 6 marks
11.
State the three observations of Rutherford's alpha-scattering experiment and the conclusion drawn from each.
12.
For the atom 3517Cl, find the number of protons, electrons and neutrons, and its valency.
Section D (5 marks each)
2 × 5 = 10 marks
13.
State Bohr's postulates and use the 2n² and outermost-shell rules to write the electronic configurations of potassium (Z = 19) and calcium (Z = 20).
14.
Explain isotopes and isobars with one example each, list one use each of U-235, Co-60 and I-131, and explain why the atomic mass of chlorine is 35.5 u.
Answer Key
Section A (1 mark each)
- (C) Chadwick
- (C) -1
- (C) 18
- (B) isobars
- (B) 2
- (B) 12
Section B (2 marks each)
- Electron: −1, ≈1/1840 u (≈0). Proton: +1, ≈1 u. Neutron: 0, ≈1 u.
- Carbon (Z = 6): 2, 4. Argon (Z = 18): 2, 8, 8.
- Isotopes are atoms of the same element with the same atomic number but different mass numbers, e.g. ¹H, ²H and ³H (hydrogen).
- It has equal numbers of protons (+1) and electrons (−1), so the charges cancel out.
Section C (3 marks each)
- Most α-particles passed straight through → the atom is mostly empty space; a few deflected by small angles → the centre has a concentrated positive charge; very few bounced back → the nucleus is very small, dense and holds nearly all the mass.
- Protons = 17, electrons = 17, neutrons = A − Z = 35 − 17 = 18. Configuration 2, 8, 7 gives valency = 8 − 7 = 1.
Section D (5 marks each)
- Postulates: electrons revolve in fixed circular orbits (shells/energy levels) of definite energy; they do not radiate energy while in a shell, so the atom is stable; energy changes only on jumping between shells. Rules: a shell holds at most 2n² electrons, the outermost shell holds at most 8, and inner shells fill first. Potassium (Z = 19): 2, 8, 8, 1. Calcium (Z = 20): 2, 8, 8, 2.
- Isotopes: same atomic number, different mass numbers, e.g. ¹H, ²H, ³H. Isobars: same mass number, different atomic numbers, e.g. ⁴⁰Ca and ⁴⁰Ar. Uses: U-235 is reactor fuel for nuclear energy; Co-60 is used in radiotherapy for cancer; I-131 treats goitre and thyroid disorders. Chlorine's atomic mass is 35.5 u because natural chlorine is about 75% ³⁵Cl and 25% ³⁷Cl, and the listed mass is the weighted average, (0.75 × 35) + (0.25 × 37) = 35.5 u, which is a fraction.
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