1. The Atomic Model Through Time
(see p in text)

2. Aristotle (460 B.C. – 370 B.C.)
emphasized that nature consisted of four elements: air, earth, fire, and water.
he did not believe in discontinuous or separate atoms, but felt that matter was continuous

3. Democritus (460 B.C. – 370 B.C.) greek philosopher
first to suggest the existence of “atoms” (Greek word “atomos” = indivisible)
atoms indivisible and indestructible
no experimental support

4. John Dalton (1766-1844) english chemist and schoolteacher
used scientific method to test Democritus’s ideas
Dalton’s atomic theory
elements composed of atoms
atoms of the same element are alike
different atoms can combine in ratios to form compounds
chemical reactions can occur when atoms are separated, joined, or rearranged (but atoms are not created nor destroyed)

5. J.J. Thompson (1856-1940) English physicist discovered the electron
thought atom was negative charges stuck in a positive charged lump
referred to as the “plumb pudding model”

6. We still use Thompson’s “cathode ray tube” in TV and other applications

7. Robert A. Millikan ( )
American physicist that continued Thompson’s work on electrons
Found the quantity of charge carried by an electron (one unit of negative charge)
Calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

8. Ernest Rutherford ( )
“Like howitzer shells bouncing off of tissue paper!”
New Zealand physicist and former student of Thompson
proposed that the atom is mostly empty space
positive charges and almost of the mass are in a small, centralized region called the nucleus

9. Evidence for Rutherford’s Conclusions
Rutherford Flash Animation

11. Try it Yourself!
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

12. The Answers
Target #1
Target #2

13. Niels Bohr (1855-1962) former student of Rutherford
electrons found only in specific circular paths (orbits) around the nucleus
based on information about how the energy of an atom changes when it absorbs and emits light
called these fixed energies “energy levels”

15. Erwin Schrodinger (1926) Austrian physicist quantum mechanical model
probability of electron locations around the nucleus
not an exact orbit
led to the electron cloud model

16. Werner Heisenberg (1927) German physicist
Heisenberg Uncertainty Principle – impossible to know the exact position and velocity of a particle (electron) at the same time
“the observer affects the observed”