1. Part 2: Rutherford and the Gold Foil Experiment
Atomic Theory

2. The Story So Far…

3. The Discovery of the Nucleus
Ernest Rutherford was a student under J.J. Thomson
Moved to Montreal to study at McGill!
Holla - Canada!

4. Rutherford’s Work Radioactive elements were just recently understood
They give off rays of energy as they break down
These rays are known as “radioactive decay”

5. Rutherford’s Work Rutherford studied alpha particles
Emitted from decaying radioactive elements (ex. Radium, Polonium)
Alpha particles are large and have a positive charge

6. Rutherford’s Work
To further study the atom, he decided to fire alpha particles at a thin sheet of gold
He expected the particles being shot at the gold would pass through in a straight line

7. Rutherford’s Work But he was surprised!
Most of the particles went straight through
Some were deflected off course (weird)
Some bounced right back at him (weirder!)
How in the world would he explain this??

8. Gold foil atoms

9. Rutherford’s Work
Since alpha particles are positive and they bounced right back at him…
Rutherford proposed that a small area of positive charge existed in the atom
The alpha particles were repelled by the like-charged centre of the gold atoms

10. Rutherford’s Work
(+)

11. Rutherford’s Work Rutherford named the centre of the atom “nucleus”
Proposed the nucleus had a positive charge
Proposed electrons orbited the nucleus, like planets around the Sun

12. Rutherford’s Work
Rutherford’s observations can be summarized as follows:
OBSERVATIONS
CONCLUSIONS 1.
Most alpha particles went straight through the foil
Atoms are mostly empty space 2.
Some alpha particles were deflected a little
The nucleus is positive and repelled the alpha particles 3.
A few alpha particles were deflected straight back
The nucleus is very tiny compared to the size of the whole atom

13. Thomson’s model vs Rutherford’s model
Major advancement in the understanding of the atom!

14. The next find…
A Danish scientist (Neils Bohr) traveled to England to work with now-famous Rutherford
Famous
Curious

15. Bohr’s Contribution Bohr figured Rutherford was mostly correct…
He passed electricity through hydrogen gas and bands of light appeared
Light bands corresponded with quantity of energy applied to the gas
Unique bands of light were given off by ALL elements

16. Bohr’s Contribution
Bohr reasoned that electrons moved in fixed regions or energy levels
For an electron to move up a level, it needed outside energy to “bump it up”
The amount of energy required to excite an electron was called a “quantum”
The discipline of quantum mechanics was born!

17. Where can electrons be found?
In concentric “shells” around the nucleus…
… but in 3 dimensions

18. Bohr-Rutherford Model of the Atom
Think of a person on a ladder:
You can’t stand between the rungs!

19. Bohr-Rutherford Model of the Atom
Electrons can be “bumped up” after absorbing energy or they can “fall down” if energy is not sustained