1. Notes #2: History of the Atom (Parts 1&2)
WCHS Chemistry

2. *** All models fall short of reality! ***
WHAT IS A MODEL?
Model = an attempt to understand or explain things
Usually things that are really big or really small
*** All models fall short of reality! ***
Over time, models become more COMPLEX! Why??
BECAUSE WE GET MORE INFORMATION!!
BIG
IDEA!!!

3. Credited for the first belief that atoms existed (atomic theory)
DEMOCRITUS ( B.C.)
Credited for the first belief that atoms existed (atomic theory)
But… NO PROOF!

4. Didn’t believe in atoms Set us back 2,000 years :(
ARISTOTLE ( B.C.)
Didn’t believe in atoms
Set us back 2,000 years :(

6. Evidence of Atoms

7. ANTOINE LAVOISIER (1770’s)
Law of Conservation of Mass
Mass before = Mass after
SO2 + H2O → H2SO3
5 grams =
5 grams

8. Law of Definite Proportions
JOSEPH PROUST (1797)
Law of Definite Proportions
Each chemical compound has a fixed and unchangeable ratio of its elements
Ex: Water
Water always has 2 hydrogen to every 1 oxygen
H2O2 = completely different compound!

9. Dalton’s Atomic Theory
JOHN DALTON (1803)
Dalton’s Atomic Theory
All matter is composed of atoms, which are indivisible and indestructible
All atoms of the same element are identical
All atoms of different elements are different
Compounds are composed of 2 or more different elements in a definite ratio

10. Law of Conservation of Mass = #1 (Lavoisier)
JOHN DALTON (1803) cont...
Theory was easily accepted because he explained what people already knew…. Just put it all together!
Law of Conservation of Mass = #1 (Lavoisier)
Law of Definite Proportions = #4 (Proust)

11. Dalton’s atomic model:
JOHN DALTON (1803)
Dalton’s atomic model:
A marble → No parts

12. Cathode Ray Tube experiment
J.J. THOMSON (1897)
Electron discovery
Cathode Ray Tube experiment
Stream of particles aimed between 2 charged plates
Particles deflected away from negatively charged place, so they must also have a negative charge (= electrons!)

13. Pudding → positively charged medium
PLUM PUDDING MODEL
Plums → electrons
Pudding → positively charged medium

14. RUTHERFORD’S “GOLD FOIL” EXPERIMENT (1909)

15. RUTHERFORD’S “GOLD FOIL” EXPERIMENT (1909)
Conclusions…
NUCLEUS
Must be really small - only 1 in 20,000 hit it
Positively charged (b/c of protons!) - positive particles bounced off
Massive (dense) - was able to make particle bounce backwards

16. Most of the atom is empty space
NEW MODEL
Most of the atom is empty space