Rutherford Model of the Atom

Contributed by:
Jonathan James
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
7. What he expected…
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!