1. Atomic Structure Democritus Evolution of Atomic theory Dalton
JJ Thomson & Millikan
Rutherford
Evolution of Atomic theory

2. Democritus (460 – 370 BC)
Teacher/Greek philosopher.
First to suggest the existence of atoms.
All matter can be broken down into indivisible atomos (atoms).

3. According to the early theory of atoms developed by Democritus, Water atoms would be round and flow over one another while iron atoms would be jagged and stay solidly together. This is not the modern view.
Democritus (460 – 370 BC)

4. This is not the modern view
Democritus (460 – 370 BC)
This is not the modern view
His ideas lacked experimental support.
They were not useful in explaining chemical behavior.
However; some of his ideas agreed with later scientific theory

5. Dalton’s Atomic Theory
( )
Englishman. School teacher
Studied the ratios in which elements combine in chemical reactions.
Based on the results of his experiments, he formulated his atomic theory

6. Dalton’s Atomic Theory
( )
Dalton’s Atomic Theory
Elements are composed of atoms.
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

7. Dalton’s Atomic Theory
( )
3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.

8. Dalton’s Atomic Theory
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4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

9. Experiments to determine the structure of an atom.
J.J. Thomson used Cathode ray tubes.

10. Thomson’s Experiment (1856-1940)
By adding an electric field, he found that the moving pieces were negatively charged.
He measured the electron’s charge to mass ratio.
And identified the electron as a fundamental particle of an atom.

11. Thomson’s Experiment (1856-1940)
Found the electron.
Said the atom was
like plum pudding.
A bunch of positive stuff, with the electrons able to be removed.

12. Rutherford’s Experiment 1907
Aimed alpha particles at gold foil by drilling a hole in a lead block.
Since the mass is evenly distributed in gold atoms, alpha particles should go straight through.
Used gold foil because it could be very thin (1atom thin).

13. What he expected
Fluorescent screen
Gold Foil

14. Particles would pass through
Because, he thought the mass was evenly distributed in the atom.

15. What he got

16. How he explained it Atom is mostly empty
Small dense, positive piece at center.
Alpha particles are deflected by it if they get close enough. +

17. What Rutherford Observed+

18. SO THERE MUST BE A SMALL MASSIVE POSITIVE CENTER!!!
THE NUCLEUS