Contributed by:
The highlights are:
1. Rutherford scattering experiment
2. Observations
3. Interferences
1.
Rutherford
Rutherford
Ernest Rutherford (1871-1937) PAPER
PAPER
• Learned physics in
J.J. Thomson’ lab.
• Noticed that ‘alpha’
particles were
sometime deflected
by something in the
air.
• Gold-foil experiment
Animation by Raymond Chang – All rights reserved.
2.
Rutherford ‘Scattering’
• In 1909 Rutherford undertook a series of experiments
• He fired (alpha) particles at a very thin sample of gold foil
• According to the Thomson model the particles would only
be slightly deflected
• Rutherford discovered that they were deflected through large
angles and could even be reflected straight back to the source
Lead collimator
Gold foil
particle
source
3.
Rutherford’s Apparatus
Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry.
beam of alpha particles
radioactive
substance
circular ZnS - coated
fluorescent screen
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
4.
Rutherford’s Apparatus
beam of alpha particles
radioactive
substance
fluorescent screen
circular - ZnS coated
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
5.
Geiger-Muller Counter
Hans Geiger
Speaker gives
“click” for
each particle
Window
Particle
path
Argon atoms
6.
Geiger Counter
Ionization of fill gas
takes place along
track of radiation (-)
Speaker gives
“click” for
each particle
(+)
Metal tube
(negatively
charged)
+ e-
Window e- +
+ + e-
e-
Ionizing Free e- are attracted to
radiation (+) electrode, completing
path the circuit and generating
a current. The Geiger
Atoms or molecules Central wire electrode counter then translates
of fill gas (positively charged) the current reading into a
measure of radioactivity.
Wilbraham, Staley, Matta, Waterman, Chemistry, 2002, page 857
8.
What he got…
richocheting
alpha particles
9.
The Predicted Result:
expected
path
marks on screen
Observed Result:
mark on
screen
likely alpha
particle path
10.
Interpreting the
Observed Deflections
. . .
.
beam of . .
. . undeflected
alpha particles
particles . .
. .
. .
. deflected particle
. .
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
11.
Rutherford Scattering (cont.)
Rutherford interpreted this result by suggesting that
the particles interacted with very small and heavy
particles
Case A Particle bounces off
of atom?
Case B
Particle goes through
atom?
Case C Particle attracts
to atom?
Case D
. Particle path is altered
as it passes through atom?
12.
Table: hypothetical description of alpha particles
(based on properties of alpha radiation)
observation hypothesis
alpha rays don’t diffract ... alpha radiation is a stream of particles
alpha rays deflect towards a negatively ... alpha particles have a positive charge
charged plate and away from a positively
charged plate
alpha rays are deflected only slightly by ... alpha particles either have much
an electric field; a cathode ray passing lower charge or much greater mass
through the same field is deflected than electrons
strongly
Copyright © 1997-2005 by Fred Senese
13.
Explanation of Alpha-Scattering Results
Alpha particles Nucleus
+
+
-
-
+
+
-
+
-
+
-
+
-
-
+
-
Plum-pudding atom Nuclear atom
Thomson’s model Rutherford’s model
14.
Results of foil experiment if plum-
pudding had been correct.
Electrons scattered
throughout positive
charges
+ -
+
- +
+ -
+
-
- +
+
+ -
-
-
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
15.
Interpreting the Observed
Deflections
deflected particle
. . .
.
beam of . .
. . undeflected
alpha particles
particles . .
. .
. .
.
. .
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
16.
Rutherford’s
Gold-Leaf
Experiment
Conclusions:
Atom is mostly empty space
Nucleus has (+) charge
Electrons float around nucleus
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
17.
• Hit moth driving car – no change in car
direction
• Hit deer – car changes direction
Alpha particle moth
Gold Atom
deer
Large angle of deflection, must have hit massive object!
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