Atomic Structure
Name : __________________
Class : F6
____(
Date : ___________________
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)
Rutherford’s Atomic Model
Multiple Choice Questions
1
Which of the following statements best describes the term ‘atom’ in physics nowadays?
A. It is the tiniest indivisible unit of matter.
B. It is the smallest unit of chemical substances.
C. It is the structure of matter which consisted of protons and neutrons.
D. It is the structure of matter which consisted of a nucleus surrounded by electrons.
Answer: D
2
Which of the following conclusions about the atomic nucleus could not be drawn from the alpha
particle scattering experiment?
(1) It consists of protons and neutrons.
(2) It is surrounded by electrons which move in circular orbits.
(3) It is very small compared with the size of the atom.
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
Answer: A
3
Which of the following conclusions about electrons could be drawn from the alpha particle
scattering experiment?
A. Electrons are negatively charged.
B. Electrons are very light compared with the atomic nucleus.
C. Electrons are concentrated at the atomic nucleus.
D. Electrons are moving in circular orbits around the atomic nucleus.
Answer: B
4
Which of the following diagrams best represents J.J. Thomson’s atomic model?
A.
B.
2
C.
D.
Answer: A
5
Which of the following diagrams best represents Rutherford’s atomic model?
A.
B.
C.
D.
Answer: C
6
Which of the following statements best describes the result of the alpha particle scattering
experiment?
A. No α-particles are deflected at large angles.
B. A few α-particles are deflected at large angles.
C. Most α-particles are deflected at large angles
D. All α-particles are deflected at large angles.
Answer: B
7
Which of the following diagrams best show the direction of the repulsive electric force FE acting on
the α particle at the position shown?
A.
B.
FE
FE
path of
α particle
nucleus
nucleus
3
path of
α particle
C.
FE
path of
α particle
D.
FE
nucleus
path of
α particle
nucleus
Answer: B
8
Which of the following statements best describe Rutherford’s atomic model?
(1) Most of the space occupied by an atom is empty.
(2) All charges inside an atom are concentrated at a small region at the centre.
(3) Electrons perform circular motion around the nucleus.
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
Answer: B
9
Which of the following best shows the size of an atom?
A. 10−5 m
B. 10−10 m
C. 10−15 m
D. 10−20 m
Answer: B
10
Which of the following best shows the size of an atomic nucleus?
A. 10−5 m
B. 10−10 m
C. 10−15 m
D. 10−20 m
Answer: C
11
Which of the following graphs best shows the number of α particles N against the deflection angle θ
obtained in the alpha particle scattering experiment?
A.
B.
C.
D.
Answer: C
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12
Which of the following particles is not discovered by scattering experiments?
A. proton
B. neutron
C. electron
D. quark
Answer: C
13
A thin gold foil is hit by a beam of α particles, in which a small number of them are deflected by an
angle larger than 90°. Which of the following can increase the number?
(1) Increase the thickness of the foil.
(2) Increase the kinetic energy of the α particles.
(3) Use a metal with a higher atomic number as the target.
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
Answer: B
14
Which of the following observations is/are inconsistent with Rutherford’s atomic model?
(1) White light produces a continuous spectrum.
(2) Accelerating charges release energy in the form of electromagnetic waves.
(3) α particles can be scattered at large angles when directing onto a thin gold foil.
A. (2) only
B. (3) only
C. (1) and (2) only
D. (1) and (3) only
Answer: A
15
In a particle scattering experiment, which of the following physical quantities of all the reactants
must be conserved?
(1) charge
(2) momentum
(3) mass-energy
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
Answer: D
16
According to Rutherford’s atomic model, the total energy E carried by an electron and the orbital
radius r of the electron from the atomic nucleus are related by
A. E  r .
B. E  r 2 .
C. E  1/ r .
D. E  1 / r 2 .
Answer: C
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17
Which of the following modifications to Rutherford’s atomic model can help explain why atoms in
gases emit electromagnetic radiation of some characteristic frequencies?
A. An atom can only contain certain numbers of nucleons.
B. An atom can only contain certain numbers of electrons.
C. Electrons orbiting a nucleus do not emit electromagnetic waves.
D. Electrons can only take certain orbits around a nucleus.
Answer: D
Structured Questions
1
Rutherford’s atomic model is also called the ‘planetary model’ of the atom. Briefly explain the one
similarity and one difference between a Rutherford’s atom and a planetary system.
(4 marks)
Answer:
Similarity:
The force between an electron and a nucleus in a Rutherford’s atom and that between a planet and a
star in a planetary system are both attractive in nature (1A). These forces provide the centripetal
forces for the electron to orbit around the nucleus and for the planet to orbit the star respectively
(1A).
Difference:
The force between the electron and nucleus is an electric force (1A) while that between the planet
and the star is a gravitational force (1A).
OR
An electron orbits around the nucleus in a circular path while a planet orbits around a star in an
elliptical path.
2
The alpha particle scattering experiment is often simulated by the following mechanical model.
bead
model hill
ramp
(a) What do the bead and model hill modelled?
(2 marks)
(b) What is the function of the ramp?
(1 mark)
(c) State two features of Rutherford’s atomic model that can be demonstrated by this mechanical
model.
(2 marks)
Answer:
(a) bead: α particle (1A); model hill: gold nucleus (1A)
(b) The ramp is to provide kinetic energy to the bead so as to simulate a fast-moving α particle in
the experiment.
(1A)
(c) In Rutherford’s model, most of the volume occupied by an atom is empty, and this is
demonstrated by the wide flattened area of the model hill (1A).
In Rutherford’s model, all the positive charge and almost all the mass of an atom is
concentrated in a small region at the centre in comparison to the whole atom, and this is
demonstrated by the relatively narrow peak of the model hill (1A).
3
(a) Describe the main features of Rutherford’s atomic model.
(3 marks)
(b) State and explain briefly two phenomena that cannot be explained by the model.(4 marks)
Answer:
(a) Most of the volume occupied by an atom is empty.
(1A)
All the positive charge and almost all the mass of an atom are concentrated in a small, dense
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region in the nucleus.
(1A)
The negatively charged electrons orbit around the nucleus in circular orbits. (1A)
(b) According to the classical electromagnetic theory, accelerating charged particles emit radiation
and lose energy (1A). Hence, in Rutherford’s model, the electrons orbiting the nucleus would
lose energy and spiral into the nucleus, leading to the collapse of an atom, but atoms do not
collapse as far as we observe (1A).
Rutherford’s model cannot explain why atoms of gas emit radiation of a set of characteristic
frequencies (1A) as it allows these atoms to emit radiation of a continuous range of frequencies
(1A).
4
In Rutherford’s atomic model, electrons orbit around the nucleus in circular orbits.
Let e be the magnitude of the electron charge, M be the mass of the nucleus, Z be the atomic
number of the nucleus, me be the electron mass and r be the orbital radius of the electron.
(a) Find the kinetic energy of the electron.
(3 marks)
(b) Find the electric potential energy of the electron.
(2 marks)
(c) In classical electromagnetic theory, an accelerating electron loses energy continuously by
emitting radiation. Explain how this affects the motion of the electron. (2 marks)
Answer:
(a) The electric force between the electron and nucleus is given by
1 Q1Q2
1 Ze   e 
1 Ze 2
FE 



(1M)
4π 0 r 2
4π 0
4π 0 r 2
r2
The electric force provides the centripetal acceleration for the orbiting electron. The kinetic
energy EK of the electron is given by
mv 2
1 Ze 2

r
4 π 0 r 2
1 2 1  1 Ze 2 

mv   
(1M+1A)
2
2  4 π 0 r 
1 Ze 2
EK 
8π 0 r
(b) The electric potential EU of the electron is
1 Q1Q2
1 Ze 2
EU 

(1M+1A)
4π 0 r
4π 0 r
(c) The total energy of the electron is
1 Ze 2
1 Ze 2
1 Ze 2
E tot  E K  EU 


(1A)
8π 0 r
4π 0 r
8π 0 r
From (b), a decrease in EK makes Etot more negative, and hence the orbital radius r of the
electron decreases.
(1A)
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5
Draw two labelled diagrams representing the path of an α particle which collides head-on with a
gold atom according to
(a) Thomson’s atomic model, and
(b) Rutherford’s atomic model.
Use darker colour to represent a higher charge density. Neglect electrons.
(4 marks)
Answer:
(a)
gold atom
α particle
(1A for correct path of α particle + 1A for correct atom)
(b)
α particle
gold nucleus
(1A for correct path of α particle + 1A for correct atom)
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