1. Ancient Greece Democritus: .
“All matter is made of tiny, unbreakable particles; atomos”
Aristotle:
“All matter is made of 4 elements; earth, air, fire and water”
~400 B. C.
Aristotelian viewpoint triumphs for 2000 years…

2. Dalton’s Atomic Theory
2000 years later…
John Dalton, a British Teacher in the late 1700’s, combined ideas of elements with that of atoms and developed
Dalton’s Atomic Theory

3. Dalton’s Atomic Theory:
All matter is made of tiny indivisible particles called atoms.
Atoms of the same element are identical.
Atoms combine to form compounds
in simple whole number ratios. (example: electrolysis)
Chemical reactions involve the rearrangement of atoms; no matter is created or destroyed.

4. 100 years later, another classy brit enters the scene:
J.J. Thompson: English Physicist (1856 – 1940) Nobel Prize in Physics 1906
Famous for:
Cathode Ray Experiments
“Discovery” of proton, neutron and electron
‘Plum Pudding’ Model of the Atom

5. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

6. Thomson’s Experiment Voltage source + -
By adding an electric field he found that the moving pieces were negative (electrons!)

7. This, and Thompson’s other experiments showed that atoms had…
positive,
(protons)
negative,
(electrons)
and neutral (neutrons)
…pieces inside them.
(but where in the atom were they?)

8. Thompson suggested a “Plum Pudding” Model of the atom:
electrons
positively
charged
“pudding” - +
It explained conduction of electricity: - +
conduction of electricity

9. Thompson’s Model of the Atom:
A bunch of positive stuff, with the electrons able to be removed
“Plum Pudding” Model;
a bunch of positive ‘pudding’,
with removable negative ‘plums’ (electrons)

10. Plum Pudding gets spilled by:
Ernest Rutherford: New Zealand Physicist ( ) Nobel Prize in Physics 1908
Famous for:
Gold Foil Experiment
“Discovery” of the nucleus
Nuclear Model of the Atom

11. Rutherford’s experiment
Took
positive charges
(alpha particles; radioactive particles that are emitted by Uranium)
and
fired
them at
Gold Atoms
His surprise:
Some of them bounced back.

12. How it worked:
When the alpha particles hit a florescent screen, the screen glows. So, he could keep track of where the positive charges ended up after he fired them at some gold foil.

13. What he expected
What he saw:

14. He Expected
The alpha particles to pass through without changing direction very much
Because
The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles…

15. Because, he thought the mass was evenly distributed in the atom

16. How he explained what he saw:+
Atom is mostly empty
Small dense, positive piece at center
Alpha particles are deflected by it if they get close enough

17. Result of Rutherford’s Gold Foil Experiment:
Result of Rutherford’s Gold Foil Experiment: “Discovery” of the Nucleus
Rutherford’s Scattering
Experiment
metal
foil
alpha
particles
Ernest Rutherford
film loop
what he expected:
ricocheting
alpha particles!
what he got:

18. Goodbye, Plum Pudding; Hello, Nuclear Model Thomson’s Atom:
diffuse mass and charge
+ alpha particles would have passed through - + ++
Hello, Nuclear Model ++
10-8 cm
10-13 cm
Rutherford’s Atom:
concentrated mass and positive charge at the nucleus
electrons roam empty space around the nucleus
some + alpha particles repelled
Prediction based on Rutherford’s nuclear atom:
number of scattered alpha particles is proportional to:
thickness of foil
square of the charge on the nucleus
1/sin4(theta/2)
1/velocity4

19. Niels Bohr: Danish Physicist (1885 – 1962) Nobel Prize in Physics 1906
Famous for:
Planetary Model of the Atom
“Father of” Quantum Theory / Mechanics
“Electrons have specific energy levels,
…like planets have specific orbits.”

20. energy levels Bohr said: n = 1 n = 2 n = 3
Electrons can only
be at certain, specific
energy
levels
(distances from the nucleus)
n = 1
n = 2
n = 3
n = the primary quantum number
(distance from nucleus

21. Electrons are found in ‘shells’ = ‘energy levels’
can hold 18 electrons
can hold 8 electrons
can hold 2 electrons

22. Valence Shell: Valence Electron:
The outermost shell that contains electrons.
Valence Electron:
An electron in an atom’s valence shell

23. Bohr Model Diagrams …show energy levels (shells) and the electrons in them. nucleus is not shown.
Inner shells fill first;
1st shell (n = 1) gets 2 e-
2nd shell (n = 2) gets 8 e-
3rd shell (n = 3) gets 8 e-
…before 4th shell starts to fill