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Rutherford’s Gold
Foil Experiment
T.T.
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Rutherford’s gold-foil experiment
Before this experiment, we assumed that:
The atom is a solid sphere
It contains negative electrons
This was called the Plum Pudding Model
Positive material
Negative electrons
Electrons were compared to fruit, positive material to cake
PDST Resources for Leaving Certificate Physics
2
Rutherford’s gold-foil experiment
2-3%
<1 %
Ra
LiS
97 %
movable Lithium sulphide detector
Rutherford fired α-particles at gold foil
He knew α-particles were small, fast moving and positive
97% went straight through
2-3% were deflected
Less than 1% bounced back
PDST Resources for Leaving Certificate Physics
3
Rutherford’s gold-foil experiment
2-3%
<1 %
Ra
LiS
97 %
movable Lithium sulphide detector
The key result is the 1% that bounce back
Rutherford compared the experiment to firing artillery shells at tissue paper
Why would any bounce back?
PDST Resources for Leaving Certificate Physics
4
Rutherford’s gold-foil experiment
+
1%
97%
+
He concluded that:
2-3%
The atom is mainly empty space, with a small area of positive charge
97% pass through because they are meeting nothing
Those deflected are coming close to a positive charge
Those that bounce back are heading straight for a positive charge
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5
Conclusions
The atom is mainly empty space
There is a small area of positive charge at the centre (the nucleus)
nucleus
+
electrons
The Rutherford – Bohr Model of the Atom (as studied at junior cert.)
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6
All elements have an atomic and mass number
e.g. for Lithium:
The total number of protons and neutrons
Mass number
7
3
Li
3p
4n
Atomic number
The number of protons
PDST Resources for Leaving Certificate Physics
7
3
Li
7