Rutherford's atom
How did he know what was inside
an atom?
Co-funded by
the Seventh Framework Programme
of the European Union
FP7-Science-in-Society-2012-1, Grant Agreement N. 321403
Geiger & Marsden's
results
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2
Draw this to scale...
1mm = 10 counts ?
Angle of
Experimental
Deflection
Count (N)
150
135
120
105
75
60
45
30
15
10
5
33
43
52
70
211
477
1435
7800
132000
503200
835400
7.0mm
2.1cm
5.2mm
4.3mm
3.3mm
14.3 cm
78 cm
83.5 m
FP7-Science-in-Society-2012-1, Grant Agreement N. 321403
3
What is the best way to
represent data?
Angle of
Experimental
Deflection
Count (N)
150
33
135
43
120
52
105
70
75
211
60
477
45
1435
30
7800
15
132000
10
503200
5
835400
• How could you
best represent
Geiger &
Marsden's results
graphically?
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4
Where do the charges live inside an atom?
(How is charge distributed within an atom?)
• Rutherford knew:
• You can extract negatively charged electrons from atoms... so
there must be positive charges inside the atom too
• Alpha particles are positively charged
and are scatered by atoms.
• Alpha particles travel quite fast (7 million m/s)
and are quite heavy, so are hard to deflect.
So Rutherford thought the charge in an atom might be...
a) Spread out
b) In lumps
c) Something else?
+
+
+
+
+
+
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?
5
What arrangement of positive charges
might explain what is seen?
• Try placing charges
(cubes) inside the
atom to see if you can
reproduce the sort of
scattering seen in
experiments
A whole Atom
Can you find a
solution?
+
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++ +
+
+
?
6
How much positive charge is
inside a atom?
•
•
•
•
For most atoms, the positive charge of the nucleus is cancelled out by the
negative charge of the electrons. It is very hard to know exactly how much
positive charge is in the nucleus.
Rutherford was able to create a mathematical model and predict how much
nuclei of different charges would deflect alpha particles.
Chadwick used the model to find out what the charge was on the nuclei of
Copper (Cu), Silver (Ag) and Platinum (Pt) atoms.
His results were:
Copper 29.3 electron charges (± 1% )
Silver 46.3 electron charges (± 1% )
Platinum 77.4 electron charges (± 1% )
Use a copy of the periodic table of 1905 to see if you
can find a link between the position in the table and the
amount of + charge in the nucleus.
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FP7-Science-in-Society-2012-1, Grant Agreement N. 321403
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What is an alpha particle?
• C.T.R. Wilson invented a cloud
chamber to view the tracks left
behind as particles pass through
a gas. He showed it to other
scientists in 1912.
• Each track is made up of
thousands of droplets, each one
formed when an alpha particle
Tracks left behind by alpha
crashes into an atom of gas.
particles.
• An electron is knocked off the
They have different amounts of kinetic
atom and it becomes charged.
energy. How can you tell this?
• A charged atom is called an "ion"
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9
Sometimes, something
odd happens!
• Can you see what?
• Electrons are 'easy' to knock off an
atom, but occasionally, the alpha
particle hits the heavy centre of a
gas atom.
• Then we get two tracks as they
bounce off each other.
• The angle they bounce off at tells
us something about how heavy
they are.
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10
How does mass affect the angles
particles bounce off each other?
• Try this: Collide two balls
• Record their tracks
• Is there a pattern?
Target ball
Moving ball
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11
Try these...
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Which gas is about the same mass
as an alpha particle?
Collision in
Nitrogen gas
Alpha particles in wet
hydrogen. One
collided with a hydrogen
nucleus, which recoiled
forward and upward,
making a thin track
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Collision
with
Oxygen
gas
Alpha particle tracks in
wet helium. One
collided with a helium
nucleus.
13
How big is the "nucleus"?
radius
Kinetic
Energy
Potential
Energy
The nucleus must
be smaller than
this radius
Kinetic
Energy
Potential
Energy
Kinetic
Energy
Alpha particle
approaching
nucleus
α
α
Nucleus
Kinetic
Energy
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A model...
radius
More
α
α
Energy of α
electric field
Launch alpha particles of different "energies" at the nucleus.
Measure how close the α gets to the centre of the nucleus.
For a 5MeV α particle, what is the biggest the nucleus can be. (Au atom)
FP7-Science-in-Society-2012-1, Grant Agreement N. 321403
15
How big is a nucleus?
Kinetic energy of alpha particle = 5 MeV
1
𝑞𝛼 𝑄𝐴𝑢
𝑟 =
𝑥
4𝜋𝜀0
𝐾𝐸 𝛼
Electron charge, e- = 1.6 x 10-19 C
𝜺𝟎 = 8.85 x 10-12
qα = 2 x eQAu = 79 x eMass alpha particle = 6.64424. 10-27 kg
Velocity of alpha particle = 15,000 km/s
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16
How big is an atom?
• Rutherford found that the nucleus was
10,000 times smaller than the outside
of the atom.
• Draw a dot about 1mm across (radius=
0.5mm) on a small round sticker.
1mm
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17
Apparatus:
Sticker
String
metre 'rule'
Use squares of tape to mark
out the outside of the atom.
Try not to overlap with any
other atoms in the room!
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18
What if we could get rid of the
space inside atoms?
10m
10m
Steel (Fe)
sphere
Atomic model
1mm
FP7-Science-in-Society-2012-1, Grant Agreement N. 321403
Radius of sphere
= 5m
Density of steel
= 7700 kg/m3
What would the
density be if this
was condensed
down to a sphere
1mm across?
19
• Summarize what you have learnt about
atoms from these activities.
• What have you learnt about the "TEMI
way" from these activities?
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20
Penalty Shoot-out
• The Geiger-Marsden Cup
How
many
can you
score
out of
10
shots?
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21
Thanks...
• Thank you to
– Emma Jones
– Abi Suter
For trialling activities and giving feedback.
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