Rutherford
Ernest Rutherford (1871-1937)
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.
Rutherford ‘Scattering’
• In 1909 Rutherford undertook a series of experiments
• He fired a (alpha) particles at a very thin sample of gold foil
• According to the Thomson model the a 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
a particle
source
q
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
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
Geiger-Muller Counter
Hans Geiger
Speaker gives
“click” for
each particle
Window
Particle
path
Argon atoms
Geiger Counter
Ionization of fill gas
takes place along
track of radiation
(-)
(+)
Speaker gives
“click” for
each particle
Metal tube
(negatively
charged)
Window
+
e-
e+
+
+ ee-
Ionizing
radiation
path
Atoms or molecules
of fill gas
Wilbraham, Staley, Matta, Waterman, Chemistry, 2002, page 857
Central wire electrode
(positively charged)
Free e- are attracted to
(+) electrode, completing
the circuit and generating
a current. The Geiger
counter then translates
the current reading into a
measure of radioactivity.
What he expected…
What he got…
richocheting
alpha particles
The Predicted Result:
expected
path
expected
marks on screen
Observed Result:
mark on
screen
likely alpha
particle path
Interpreting the
Observed Deflections
.
.
.
.
.
.
beam of
alpha
particles
.
.
.
.
.
undeflected
particles
.
.
.
.
.
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
.
deflected particle
Rutherford Scattering (cont.)
Rutherford interpreted this result by suggesting that
the a particles interacted with very small and heavy
particles
Particle bounces off
of atom?
Case A
Case B
Particle goes through
atom?
Particle attracts
to atom?
Case C
Case D
.
Particle path is altered
as it passes through atom?
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
charged plate and away from a positively
charged plate
... alpha particles have a positive charge
alpha rays are deflected only slightly by
an electric field; a cathode ray passing
through the same field is deflected
strongly
... alpha particles either have much
lower charge or much greater mass
than electrons
Copyright © 1997-2005 by Fred Senese
Explanation of Alpha-Scattering Results
Alpha particles
Nucleus
+
+
-
-
+
+
-
+
+
-
+
-
+
-
-
Plum-pudding atom
Nuclear atom
Thomson’s model
Rutherford’s model
Results of foil experiment if plumpudding had been correct.
Electrons scattered
throughout
-
+
-
positive
charges
+
+
+
+
-
-
+
+
+
-
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57
-
Interpreting the Observed
Deflections
deflected particle
.
.
.
.
.
.
beam of
alpha
particles
.
.
.
.
.
.
.
.
.
.
gold foil
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120
.
undeflected
particles
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
• 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!