1. The Atomic Theory Atomic Theory Matching Atom Size Democritus
Dalton – Ball Model
J.J. Thompson – Raisin Bun Model
Rutherford – Gold Foil Experiment
Bohr – Energy Level Model

2. How are atoms studied? Atoms are the building blocks of matter
Atoms are too small in size to study easily
Size of Earth : soda can = soda can : atom

3. Sizing up the Atom
Materials are able to be subdivided into smaller and smaller particles – these are atoms.
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long

4. The Size of the Atom
Despite their small size, individual atoms are observable with instruments such as scanning electron microscopes
In this picture, atoms are being moved by the single atom tip of a Atomic Force Microscope (AFM). Apart from allowing scientist to image atoms, this instrument also allows them to actually move them one at the time.

5. The Atom is Really, Really Small!
Just How Small Is An Atom?

6. The Greek philosopher Democritus (460 B. C. – 370 B. C
The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms
He believed that atoms were indivisible and indestructible

7. Dalton’s Atomic Theory – The Ball Model: 1800s
Referred to the atom as a small, hard, indestructible sphere that cannot be subdivided
Different atoms have different properties
An atom is the smallest particle of an element
an element is a substance made up of only 1 type of atom

8. J.J. Thomson
In 1897 J.J. Thomson demonstrated that there are negatively charged particles found within atoms using cathode ray tubes
After further investigation he concluded that the same negatively charged particles are found in all atoms
These are now called electrons (e-)

10. Thomson’s Model He knew atoms didn’t have a charge (+ or -)
Since his particles were negative, he knew that there must also be positively charged particles within the atom
J. J. Thomson

11. Thomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a positively charged “pudding”
It was called the “Plum Pudding” or “Raisin Bun” model.

12. Ernest Rutherford’s Gold Foil Experiment - 1911
“+” charged particles (alpha particles) were fired at a thin sheet of gold foil
Particles that hit on the detecting screen (blue film) were recorded

13. What he thought would happen given the “Plum Pudding” Model
Some alpha particles would be slightly diverted due to their avoidance of the other positive particles inside the atom

14. What he actually found
Most of the alpha particles slightly deflected and carried on but some bounced backwards
Therefore, they must have collided and rebounded off something very dense

15. Rutherford’s Gold Foil Experiment

16. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected (1 out of 8000!)
Egads! It’s like shooting bullets at tissue paper and watching them bounce back

17. The Rutherford Atomic Model
Based on his experimental evidence he concluded that the atom:
Is mostly empty space
Contains a small, dense, positively charged nucleus at the center
Negatively charged electrons revolved around the positive nucleus

18. Bohr
Niels Bohr
If there are negatively charged particles around a positively charged nucleus, why don’t they attract and collide (opposites attract) - - + - -

19. The Bohr-Rutherford Model
Central positive nucleus
Electrons move in orbits (energy levels) around the nucleus
Electrons more or less stay in their orbit unless an increase in energy causes them to move