5.3.1 The Nuclear Atom
(a) describe qualitatively the alpha-particle
scattering experiment and the evidence this
provides for the existence, charge and
small size of the nucleus
(HSW)
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Alpha Particle Scattering
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Gold Foil Experiment
What were Geiger and Marsden’s results?
2. Some alpha particles
were slightly deflected
by the gold foil.
3. A few alpha
particles were
bounced back
from the gold foil.
1. Most alpha particles
went straight through
the gold foil, without
any deflection.
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How did Rutherford interpret the results?
Rutherford had expected all the alpha
radiation to pass through the gold foil. He
was surprised that some alpha particles
were deflected slightly or bounced back.
The ‘plum pudding’ model could not explain
these results, so Rutherford proposed his
‘nuclear’ model of the atom.
He suggested that an atom is mostly
empty space with its positive charge and
most of its mass in a tiny central nucleus.
Electrons orbited this nucleus at a
distance, like planets around the Sun.
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Gold Foil Experiment
The experiment was carried out in a vacuum, so
deflection of the alpha particles must have been due
to the gold foil.
How can these results be explained in terms of
atoms?
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Gold Foil Experiment
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Video
http://www.youtube.com/watch?v=u0HMYLSUzTU
(b) describe the basic atomic structure of
the atom and the relative sizes of the atom
and the nucleus
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Structure and size of the atom
Using Page 181 in Physics 2:
 describe the basic atomic structure of the atom
 and the relative sizes of the atom and the nucleus
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Structure and size of the atom
Atom:
 protons and neutrons make up the nucleus of the atom
 the electrons move around the nucleus in a cloud,
some closer to and some further from the centre of the
nucleus
Scale:
 radius of proton ~ 10-15m
 radius of proton ~ 10-15m
 radius of nucleus ~ 10-15m to 10-14m
 radius of atom ~ 10-10m
 size of molecule ~ 10-10m to 10-6m
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Structure and size of the atom
… or …
 if the nucleus was a grape seed in the centre circle, the
electrons would be orbiting around the outside of
Wembley Stadium
… or …
if the nucleus was the size of a marble, the orbiting
electron would be a grain of sand 800m away
… or …
 if the nucleus was a shopping trolley in Trafalgar
Square, the electrons would be orbiting around the M25
(c) select and use Coulomb’s law to determine the force
of repulsion, and Newton’s law of gravitation to
determine the force of attraction, between two protons at
nuclear separations and hence the need for a short
range, attractive force between nucleons (HSW)
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Forces in the nucleus
 Nucleus consists of protons (+e) and neutrons
 Logic dictates that the +e protons should repel each
other
 The fact that they don’t indicates another force in the
nucleus that holds the nucleons together
 This force is called the strong nuclear force
 It only acts over small (10-14m) distances
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Forces in the nucleus
 There are two other forces in the nucleus:
 electrostatic
 gravitational
 The strength of each force can be calculated by:
 repulsive electrostatic – Coulomb’s law
 attractive gravitational – Newton’s laws
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Forces in the nucleus
Two protons in the nucleus of an atom are separated
by 1.6 x 10-15 m.
Calculate the force of electrostatic repulsion between
them, and the force of gravitational attraction between
them.
2. Is the force of gravity enough to balance the electric
repulsion tending to separate them?
3. What does this suggest to you about the forces
between protons in the nucleus?
1.
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Forces in the nucleus
Coulomb’s law:
where Q and q
r
ε0
= point charges
= distance between point charges
= permittivity of free space
Newton’s law of gravitation:
where M and m
r
G
F = GMm
r2
= mass of each object
= distance between each object
= gravitational constant
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Forces in the nucleus
Data:
Proton
Q = 1.610-19C
m = 1.6710-27kg
ε0 = 8.8510-12 Fm-1
G = 6.67 x 10-11 Nm2kg-2
(d) describe how the strong nuclear force
between nucleons is attractive and very shortranged
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Strong Nuclear Force
 Acts over very short distances (10-14m)
 Attractive force
 Only reaches adjacent nucleons
 Large nucleus not held together as tightly as a small
nucleus
(e) estimate the density of nuclear matter
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Density of Nuclear Matter
Mass of proton mp = 1.67 x 10-27 kg
Radius of proton r = 0.80 x 10-15 m
Volume of proton = 4 πr3
3
Density (kg m3)
= mass (kg)
volume (m3)
(f) define proton and nucleon number
(g) state and use the notation for the
representation of nuclides
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Nucleons and electrons
Nucleon number:
The number of
protons plus the
number of neutrons in
a neutral atom.
Proton number:
The number of
protons (which is the
same as number of
electrons in a neutral
atom).
A
X
Z
Element
symbol
(h) define and use the term isotopes
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Isotopes
All carbon atoms have the same number of
protons, but not all carbon atoms are identical.
Although atoms of the same element always have
the same number of protons, they can have
different numbers of neutrons. Atoms that differ in
this way are called isotopes.
For example, carbon exists as three different
isotopes: carbon-12, carbon-13 and carbon-14:
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Isotopes
For example, carbon exists as three different
isotopes: carbon-12, carbon-13 and carbon-14:
Nucleon
number is
different
Proton
number is
the same
Potassium is another element that exists as three
different isotopes: potassium-39, potassium-40 and
potassium-41.
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Isotopes
Isotopes are nuclei of the same element with a
different number of neutrons but the same number
of protons.
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Assessment
 Chapter 12 SAQ’s 1 to 11
 End of Chapter 12 questions 1 - 3
 Atomic Structure worksheet questions 1 – 5 and 8 – 10
(i) use nuclear decay equations to represent
simple nuclear reactions
(j) state the quantities conserved in a nuclear
decay.