1. Early Ideas About Matter
The History of the Atom
Early Ideas About Matter

2. The Early Greek Philosophers
Democritus
( B.C.)
Matter is composed of atoms (atomos), which move through empty space.
Atoms are solid, homogeneous, indestructible, and indivisible.
Different kinds of atoms have different sizes and shapes.
Size, shape and movement of atoms determine the properties of matter.
Criticism- “What holds atoms together?”

3. The Early Greek Philosophers
Aristotle
( B.C.)
Disagreed with Democritus.
He did not believe that atoms move through empty space.
Used his influence to “disprove” Democritus.
Felt that all matter was made of either Earth, Wind, Fire or Water.
Unchallenged ideas for 2,ooo years!

4. Aristotle’s Elements Earth, Wind & Fire

5. The “English Revolution”
John Dalton
( )
British Schoolteacher
Based on his research, revived Democritus’ ideas.
Studied various chemical reactions and developed his own atomic theory in 1803.
In addition to Democritus’ thoughts, he said different atoms combine in simple, whole # to form compounds.
In a reaction, atoms are separated, combined or rearranged.

6. The Law of Conservation of Mass

7. How big is an atom? If we make an atom the size of an orange….
……then an orange would be the size of The Earth!

8. What’s inside an atom? British Physicist
J.J. Thompson ( )
British Physicist
Performed cathode-ray experiments.
The first to discover that there were particles smaller than atoms- electrons.
Determined the charge-to-mass ratio of electron.
Won Nobel Prize in 1906.
More importantly, it led to the invention of the television.
Cathode Ray Experiment

9. What’s inside an atom? American Physicist Famous Oil Drop Experiment
Robert Millikan ( )
American Physicist
Famous Oil Drop Experiment
Oil sprayed in a chamber and x-rays charged the oil.
By controlling the drops of oil in a magnetic field, he found the charge of an electron.
Used Thompson’s work and discovered the mass of an electron to be 1/1840th of a hydrogen atom.

10. The Plum Pudding Model
The chocolate chip muffin model is a more modern “plum pudding model.” The chips are like the electrons.

11. More than just electrons….
Ernest Rutherford( )
American Physicist
Gold Foil Experiment
Bombarded gold foil with radioactive alpha particles.
By following where the alpha particles were deflected, he “discovered” that there was “something” inside an atom. It wasn’t electrons scattered throughout.
Discovered the “nucleus” saying the alpha particles moved like firing a cannon at a sheet of paper and having the cannon shell coming back to you.

12. Rutherford’s Contribution
The Gold Foil Experiment
Rutherford’s Model of Atom
A little different than plum pudding. More like a hostess cupcake.

13. Subatomic Particles Particle Symbol Location Relative Charge
Relative Mass
Actual Mass (g)
Electron e-
In the space surrounding the nucleus 1-
1/1840
9.11 x 10-28
Proton p+
In the nucleus 1+ 1
1.673 x 10-24
Neutron N
1.675 x 10-24