1. Models of the Atom a Historical Perspective

2. Greek: 400 B.C. - Democritus
thought matter could not be divided indefinitely.
He named the smallest piece of matter “atomos,” meaning “not to be cut.”
Democritus

3. Greek: 350 B.C - Aristotle
changed an earlier theory that matter was made of four “elements”: earth, fire, water, air.
Aristotle was wrong. However, his theory persisted for 2000 years.
Aristotle
fire
air
water
earth

4. John Dalton (Atomic Theory)
1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine in constant ratios to form compounds.
Atoms are rearranged in reactions.
His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

5. 1897- J.J. Thomson (The electron)
“Plumb Pudding Model”
Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

6. 1897- J.J. Thomson
Thomson studied the passage of an electric current through a gas.
As the current passed through the gas, it gave off rays of negatively charged particles.

7. 1908- Ernest Rutherford (nucleus)
Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance
path of invisible -particles
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass.

8. Rutherford Scatting Experiment