Vidaara.orgClass 11 · Chemistry
CodeVID-C11-02-T1-01
Sub-atomic Particles & Atomic Models — Assignment
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 discoverer of the neutron was:
- A.J. J. Thomson
- B.Rutherford
- C.Chadwick
- D.Millikan
2.
Number of neutrons in $^{37}_{17}\text{Cl}$ is:
- A.17
- B.18
- C.20
- D.37
3.
$^{14}_{6}\text{C}$ and $^{14}_{7}\text{N}$ are:
- A.isotopes
- B.isobars
- C.isotones
- D.isomers
4.
The radius of the $n$th Bohr orbit is proportional to:
- A.$n$
- B.$n^2$
- C.$1/n$
- D.$1/n^2$
5.
The Lyman series of hydrogen lies in the:
- A.visible region
- B.ultraviolet region
- C.infrared region
- D.microwave region
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Distinguish between isotopes and isobars with one example of each.
7.
State the two main observations of the gold-foil experiment and what each implies.
8.
Why does a hydrogen atom in its ground state not radiate energy, according to Bohr?
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Calculate the radius and energy of the second Bohr orbit of hydrogen ($r_1=0.529\,\text{angstrom}$).
10.
Compute the wavelength of the first line of the Lyman series ($n_2=2\to n_1=1$), $R_H=1.097\times10^{7}\,\text{m}^{-1}$.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
State Bohr's postulates for the hydrogen atom and use them to explain the origin of the line spectrum. List two limitations of the model.
Answer Key
Section A — Multiple Choice Questions
- (C) Chadwick
- (C) 20
- (B) isobars
- (B) $n^2$
- (B) ultraviolet region
Section B — Short Answer (2 marks)
- Isotopes: same $Z$, different $A$ (e.g. $^{12}_{6}\text{C}$, $^{14}_{6}\text{C}$). Isobars: same $A$, different $Z$ (e.g. $^{40}_{18}\text{Ar}$, $^{40}_{20}\text{Ca}$).
- Most $\alpha$-particles pass undeflected (atom is mostly empty space); a few are strongly deflected/backscattered (tiny dense positive nucleus).
- The $n=1$ orbit is a stationary state of fixed energy; an electron in it does not accelerate-radiate, and no lower state exists to fall to.
Section C — Short Answer (3 marks)
- $r_2=0.529\times4=2.116\,\text{angstrom}$; $E_2=-13.6/4=-3.40\,\text{eV}$.
- $1/\lambda=R_H(1-1/4)=R_H\times0.75=8.23\times10^{6}\,\text{m}^{-1}$; $\lambda\approx1.215\times10^{-7}\,\text{m}=121.5\,\text{nm}$.
Section D — Long Answer (5 marks)
- Postulates: stationary orbits (no radiation), quantised angular momentum $mvr=nh/2\pi$, energy change only on transitions $\Delta E=h\nu$. Lines arise from electrons falling between fixed levels, each $\Delta E$ giving a fixed $\nu$/$\lambda$ (Rydberg formula). Limitations: works only for one-electron species; cannot explain fine structure / Zeeman-Stark splitting / ignores wave nature.
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