1. Chapter 4 Atomic Structure
4.1 Studying Atoms

2. Early Chemists and Atomic Models
Who are the early chemists that shaped atomic theory?
What were the accepted models of the atom?
How have the models changed over time?
What experiments were performed to change the models?

3. Ancient Greek Models
Democritus was one of first philosophers on record
Believed that matter was made of extremely small particles
Different particles had different properties

4. Ancient Greek Models
Aristotle was another Greek that described atomic structure
His model was that of the 4 elements :
Fire -Air
Water -Earth

5. Dalton’s Atomic Theory
Dalton studied the behavior of gases in air
Discovered that gases consist of individual particles instead of “one substance”

6. Evidence for Atoms
Measured mass of elements when they formed compounds
Showed that the ratio of masses was always the same

7. Example 16 g of Oxygen gas reacts with 24 g of Magnesium
Ratios are always the same

8. Dalton’s Theory All elements are composed of atoms
All atoms of the same element have the same mass
Compounds contain atoms of more than one element
Atoms always combine the same way in each compound

9. Dalton’s Theory Atoms are represented by spheres
Atoms of different elements would be different size spheres

10. Thomson’s Model
Thomson noticed that materials attract or repel other materials when rubbed
Like charges repel, unlike attract
Some charged particles can flow from one location to another

11. The Experiment
Video
Thomson used a sealed glass tube with an electric current
A glowing beam is generated between the metal plates at the ends

12. The Experiment
Thomson placed charged particles on either side of the glass tube
The stream of particles would bend toward or away from the charged plates

13. Evidence for Subatomic Particles
The particles were negative because they were repelled by negative charged plates
All metal disks reacted the same way, so he assumed the particles came from inside atoms
Their mass was 1/2000 the mass of Hydrogen atoms

14. The Conclusions
Thomson concluded that Dalton was wrong about atoms being the smallest particles
His experiments showed that atoms were made of smaller things with charges

15. Thomson’s Model Referred to as the “Plum Pudding” model
Since atoms are neutral, the charges needed to be spread out
Negative charges were evenly distributed among the positive particles of the atom

16. Earnest Rutherford He lived from 1871-1937
He was testing some of the ideas of Thomas with gold foil
He was working with Uranium, which emits a fast moving particle with a positive charge

17. The Hypothesis
Rutherford believed that the mass and charge of particles in gold would be too small to deflect the alpha particles of Uranium
The particles would pass straight through the gold or only move slightly

18. The Experiment A stream of alpha particles was aimed at gold foil
The particles would hit a screen that was placed around the foil and would flash when hit
The location of the flash was studied

19. The Result
One out of every 20,000 particles would deflect by more than 90˚
Some particles bounced straight back

20. The Conclusion Something was making the particles deflect
The charge was not evenly distributed
The positive charged particles were concentrated in one place

21. The Nucleus
The nucleus is the location of all of the positive charges in the atom
Most of an atom is empty space where the electrons are located