1. History of the Development of the Modern Model of the Atom

2. Dalton’s Atomic Theory (early 1800’s)
All elements are composed of indestructible particles called atoms
All atoms of an element are exactly alike
Atoms of one element (ex. C) are different from atoms of other elements (ex. U)
Compounds are formed by joining two or more different atoms in constant whole number ratios.

3. What did Dalton think the atom looked like?

4. Dalton was not quite correct.
atoms are NOT indestructible particles
All atoms of an element are NOT exactly alike

5. Thomson and the Cathode Ray Tube (late 1800’s)
Thomson concluded that the atom was made of smaller particles.
Discovered particles with negative charge
Electron
No matter what metals were used they all produced a negative beam of light
Therefore atoms of all elements contain electrons

6. Video
Cathode Ray Tube
Animation (53 sec)
Demo (2:48)

7. Thomson theorized a new model of the atom.
Very dense with negative particles and positive space between.

8. Rutherford’s Gold Foil Experiment

10. OBSERVATIONS
most  particles went straight through
few  particles were deflected or bounced back
CONCLUSIONS
atoms are mostly empty space
few  particles came close to or hit a positive nucleus in the atom

11. Rutherford’s Model
Electrons randomly orbit a large, positive nucleus

12. Bohr Model of the Atom (early 1900’s)
Electrons have definite, fixed orbits
Electrons have a definite energy
orbits farther from the nucleus have greater energy

13. How many
e- in each?
Energy Level 2
Energy Level 1

14. Electron-cloud model Modern Model
electrons are most likely found in
regions outside the nucleus called orbitals
and do NOT travel in fixed paths

15. orbital shapes