1. Atomic Theory
Model
A familiar idea used to explain unfamiliar facts observed in nature
Theory
An explanation of observable facts and phenomena
To remain valid, models and theories must:
Explain all known facts
Enable scientist to make correct predictions
Episode 301

2. Proposed the existence of an atom
Democritus
Proposed the existence of an atom
Word comes from the Greek word atomis which means not to cut or “indivisible”
Aristotle
Rejected the idea of the atom
Said matter could not be cut continually
Episode 301

3. John Dalton’s Theory proposed that atoms
Atoms are the building blocks of matter
Atoms are indivisible
Atoms of the same element are identical
Atoms of different atoms are different
Atoms unite in small whole number ratios to form compounds
Episode 301

4. J. J. Thomson
Credited with discovery of electron: a blow to Dalton’s indivisible atom
Cathode Ray – negatively charged stream of particles (electrons)
Proposed the plum pudding model of the atom: negatively charged electrons embedded in a ball of positive charge
Episode 301

5. Ernest Rutherford’s Gold-foil Experiment
Aimed alpha particles at gold foil
Most passed through
A few particle were deflected
Some particles were even bounced back
Conclusions to Rutherford’s experiment
Most of the atom is empty space
Dense positively charged nucleus
Planetary model
Episode 301