1. History of the Atom

2. History of the atom Not the history of atom, but the idea of the atom.
Original idea Ancient Greece (400 B.C.)
Democritus - Greek philosopher.

3. History of Atom Looked at beach Made of sand Cut sand - smaller sand
Smallest possible piece?
Atomos - not to be cut

4. Another Greek Aristotle - Famous philosopher
All substances are made of 4 elements
Fire - Hot
Air - light
Earth - cool, heavy
Water - wet
Blend these in different proportions to get all substances

5. Who Was Right? Did not experiment.
Greeks settled disagreements by argument.
Aristotle was a better debater - He won.
His ideas carried through middle ages.
Alchemists tried to change lead to gold.

6. Who’s Next? 1803 - John Dalton- England.
Teacher- summarized results of his experiments and those of others.
Elements substances that can’t be broken down
In Dalton’s Atomic Theory
Combined idea of elements with that of atoms.

7. Dalton’s Atomic Theory
All matter is made of tiny indivisible particles called atoms.
Atoms cannot be destroyed. In chemical reactions, the atoms rearrange, but do not break apart.
All atoms of the same element have the same properties and identical masses.
Atoms of different elements have different properties and masses.
When atoms of one element combine with the atoms of another element, they always combine in the same proportional ratio.

8. Dalton’s Theories Supported
Law of Definite Proportion
(Chemical Composition)
Law of Conservation of Mass
Law of Multiple Proportions

9. Law of Definite Proportions (Law of Chemical Composition)
Each compound has a specific ratio of elements.
It is a ratio by mass.
Water is always 8 grams of oxygen for each gram of hydrogen.

10. Law of Conservation of Mass
Mass is neither created or destroyed in a chemical reaction.

11. Law of Multiple Proportions
If two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each, is a simple whole number.
The ratio of the ratios is a whole number.

12. What? Water is 8 grams of oxygen per gram of hydrogen.
Hydrogen peroxide is 16 grams of oxygen per gram of hydrogen.
16 to 8 is a 2 to 1 ratio.
True because you have to add a whole atom, you can’t add a piece of an atom.

13. Parts of Atoms J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.
A limited amount of other gases are put in

14. Thomson’s Experiment
Voltage source - +
Metal Disks

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

16. Thomson’s Experiment
Voltage source + -
By adding an electric field

17. Thomson’s Experiment Voltage source + - By adding an electric field
By adding an electric field he found that the moving pieces were negative

18. Thomson’s Experiment
Used many different metals and gases
Beam was always the same
Ray was deflected if a magnetic or electrical field were brought near
Found that cathode ray would bend toward the positive pole
 The cathode rays were negatively charged; were called electrons

19. Thomsom’s Model Discovered the electron.
Model shows negatively charged electrons embedded into an atom filled with positively charged material
Said the atom was like plum pudding.

20. Millikan’s Experiment - 1909
Atomizer - +
Metal Plates
Oil
Microscope

21. Millikan’s Experiment
Atomizer
Oil droplets - +
Oil
Microscope

22. Millikan’s Experiment
X-rays
X-rays give some drops a charge by knocking off electrons

23. Millikan’s Experiment+ -

24. Millikan’s Experiment- - + +
They put an electric charge on the plates

25. Millikan’s Experiment- - + +
Some drops would hover

26. Millikan’s Experiment- - - - - - - +
Some drops would hover + + + + + + +

27. Millikan’s Experiment- - + +
From the mass of the drop and the charge on
the plates, he calculated the charge on an electron

28. Rutherford’s Experiment
Ernest Rutherford English physicist. (1911)
Believed the plum pudding model of the atom was correct.
Wanted to see how big they are.
Used radioactivity.
Alpha particles - positively charged pieces given off by uranium.
Shot them at gold foil which can be made a few atoms thick.

29. Rutherford’s experiment
When the alpha particles hit a florescent screen, it glows.

30. Flourescent
Screen
Lead block
Uranium
Gold Foil

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

32. What he expected

33. Because

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

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

36. What he got

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

38. +

39. Density and the Atom
Since most of the particles went through, it was mostly empty.
Because the pieces turned so much, the positive pieces were heavy.
Small volume, big mass, big density.
This small dense positive area is the nucleus.

40. Niels Bohr
Applied quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits around the nucleus.
Often referred to as the “Planetary Model”
This led to the calculation of possible
energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit.

41. Louis de Broglie
Thomson’s experiments demonstrated that electrons act like particles that have mass.
de Broglie pointed out that the behavior of electrons was similar to the behavior of waves. (like light)
Frequencies could correspond to the specific energy levels associated with the Bohr model.

42. Modern View The atom is mostly empty space. Two regions.
Nucleus- protons and neutrons.
Electron cloud- region where electrons are located in areas of high probability called orbitals

43. Other pieces
Proton - positively charged pieces 1840 times heavier than the electron. (Discovered in 1914 by Henry Moseley)
Neutron - no charge but the same mass as a proton. (Discovered by James Chadwick in 1932)
Where are the pieces?