1. Chapter 8 The Composition of the atom
Atomic Theory
Chapter 8
The Composition of the atom

2. …In the Beginning…
Ancient Greece
Circa 400 BC
“Democritus”

3. Democritus concluded...
Matter is composed of Indivisible, invisible, tiny pieces.
“Atomon”=Indivisible

4. Early Theories of Matter
The Great Aristotle
He Criticized Democritus and did not believe his ideas!
People believed Aristotle because he was a great philosopher. As a result, his view was unchallenged for 2000 years!
Democritus and Aristotle just had ideas, not scientific proof. They could not test if it was valid. He was the first to propose the term “atom” to matter

5. Let There Be….nothing significant for 1700 years
Chemistry was definitely present
Chinese Empire
Dark Ages

6. 1803 John Dalton “Law of Definite Composition” A British Schoolteacher
First to apply Democritus’ idea of the atom to chemistry
He noted that in experiments where compounds were formed, the elements always combined in the same percentage by mass
What was this idea called again?
“Law of Definite Composition”
Proved that this law could be explained if matter is composed of atoms

7. 1803 John Dalton Dalton’s Model of the Atom:
Uniform Shape
Uniform Density
Indivisible
So what did Dalton’s model look like?
A simple design that promoted future research into atomic theory…

8. Dalton’s Atomic Theory
1. All matter is made up of extremely small particles called atoms.
2. All atoms of a given element are identical, having the same size, mass and chemical properties. Atoms of a specific element are different from those of any other element.
3. Atoms cannot be created, divided into smaller particles, or destroyed.
4. Different atoms combine in simple whole-number ratios to form compounds.
5. In a chemical reaction, atoms are separated, combined, or rearranged.

9. Was all of Dalton’s Theory accurate?
1. NO! His theory had to be revised as additional info was learned that could not be explained by his theory.
2. He was wrong about atoms being indivisible! They are divisible into several subatomic particles!
3. He was mistaken about atoms of a given element having identical properties. Atoms of an element may have slightly different masses.

10. The Cathode Ray Tube (CRT)
Developed by Sir William Crookes in the late 1800s

11. Thank you Sir William Crookes-for your Accidental Discovery!
Discovery of cathode rays led to invention of the TV!!!!
TV and computer monitor images are formed as radiation from the cathode strikes the light producing chemicals that coat the backside of the screen.

12. Discoveries from the CRT
Wilhelm Roentgen (1895) ?

13. Henri Becquerel 1896
Roentgen’s Discovery of X-rays provoked further research by many others, including Becquerel
Since fluorescent materials could be charged by X-rays, Henri wondered if naturally fluorescent minerals could give off X-rays….
Like any good scientist, he set up a test…

14. Photographic film sealed away
Henri Becquerel 1896
Photographic film sealed away

15. Henri Becquerel 1896 The photographic paper was still exposed!
The mineral contained Uranium and was
RADIOACTIVE

16. Electromagnets (turned off)
JJ Thompson 1897
Using the CRT…
Fluorescent Screen
Electromagnets (turned off)
Cathode (--)
Anode (+)
Beam of “light”

17. JJ Thompson 1897
When the magnets were turned on, the beam was pulled towards the + plate
What 2 things could Thompson conclude about the beam?
+ side
-- side

18. J. J. Thomson 1890’s
J.J. proposed a model of the atom known as the “plum pudding model.”
Thomson’s model consisted of a spherically shaped atom made up of uniformly distributed positive charge within which the negatively charged particles resided.

19. AKA The Chocolate-Chip Cookie Model
JJ Thompson 1897
What did his experiment discover?
ELECTRONS! (negatively-charged particles)
How did the model of the atom change since Dalton?
AKA The Chocolate-Chip Cookie Model

20. Robert Millikan 1909
Determined the charge of an electron.
Mass of an electron = 9.1 x g
Mass of an electron is extremely small.

21. Ernst Rutherford 1898
After the discoveries of Becquerel and Thompson, Rutherford was dissatisfied with the atomic models and set out to disprove it.
Using the recent discovery of Radiation (from Bacquerel), he and his lab assistants (Geiger and Marsden) set out…

22. Ernst Rutherford Early 1900s

23. Ernest Rutherford 1911
Studied how positively charged alpha particles interacted with solid matter.
Conducted an experiment to see if alpha particles would be deflected as they passed through a thin foil of gold. Also known as Rutherford’s Gold Foil Experiment.

24. Ernest Rutherford’s Gold Foil Experiment
1. A narrow beam of alpha particles was aimed at a thin sheet of gold foil.
2. A zinc sulfide coated screen surrounding the gold foil produced a flash of light whenever it was struck by an alpha particle.

25. Rutherford’s Gold Foil Experiment Results
1. Most all of the alpha particles passed straight through the gold foil, without deflection.
2. Some alpha particles were scattered at small angles.
3. A few times they were deflected at very large angles.

26. Rutherford’s Gold Foil Conclusions
1. Rutherford said plum pudding model was incorrect.
2. An atom consisted mostly of empty space through which the electrons move.
3. There was a tiny, dense, positive region, called the nucleus, centrally located within the atom that contained all of an atom’s positive charge and almost all of its mass.

27. Rutherford’s Gold Foil Conclusions
1. If a nucleus were the size of the dot at the end of this sentence, its mass would be as much as that of 70 cars!
2. Rutherford’s atomic model: most of an atom consists of electrons moving rapidly through empty space. The electrons move through available space and are held within the atom by their attraction to the positively charged nucleus!

28. Rutherford’s Completing the Atom
How did the model of the atom change?
What should it look like based on Rutherford’s conclusions?

29. Summary
Models of the atom:
Dalton Thompson
Rutherford Modern