1. Early Greeks to Present
The History of the Atom
Early Greeks to Present

2. Democritus (460 B.C.-370 B.C.) Greek Philosopher
Among the first to suggest the existence of atoms.
Believed that atoms were indivisible and indestructible.
No experimental support.
Ideas not based on scientific method.

3. Aristotle (384 B.C.-322 B.C.) Greek Philosopher
Believed all matter was made up of atoms.
Gave matter the name hyle.
No experimental data.

4. John Dalton (1766-1844) English Chemist and School Teacher
Used experimental methods.
Transformed Democritus’s ideas on atoms into a scientific theory.
Responsible for Dalton’s Atomic Theory.

5. Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of one element are different from those of any other element.
Atoms of different elements can physically combine in simple whole-number ratios to form compounds.
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element.

6. J. J. Thomson. (1856-1940) Excuse me
J.J. Thomson ( ) Excuse me... how can you discover a particle so small that nobody has ever seen one?
1897-English Physicist
Discovered the electron.
Used cathode ray tube experiment to explain his theory.

7. Cathode-Ray Tube

8. Cathode Ray-Tube (Crooke’s Tube)

9. Cathode-Ray Tube Experiment
Thomson performed experiments that involved passing electric current thru gases at low pressure.
He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes.
Electrodes were connected to a source of electricity.
Two ends of the tube
1. cathode- negatively charged electrode.
2. anode- positively charged electrode.
Cathode Ray- glowing beam that travels from the cathode to the anode.

10. Cathode Ray Tube Experiment Continued…
The beam traveled across the tube from the cathode to the anode.
The beam was deflected by negative charges, but was attracted to positive charges.
He concluded that the particles that made up the beam must be negatively charged. He called them electrons.

11. Plum Pudding Model J.J. Thomson proposed this model of the atom.
Electrons were stuck into a lump of positive charge, similar to raisins stuck in dough.

12. Robert A. Millikan (1868-1953) U.S. Physicist
1916- Calculated the mass of electron.
Values for electron charge and mass reported in 1916 are very similar to those accepted today.
Electrons carry exactly one unit of negative charge.
Electrons have a mass of 1/1840 the mass of a hydrogen atom.

13. Eugen Goldstein (1850-1930) German Physicist
Observed rays traveling in the direction opposite to that of the cathode rays.
He called these rays canal rays and concluded they were positively charged.
Now called protons.
Mass of a proton is 1840 times that of an electron.

14. James Chadwick (1891-1974) English Physicist
1932-Confirmed the existence of another particle with no charge- neutron.

15. Ernest Rutherford (1871-1937) Student of Thomson
1911- Tested the theory of atomic structure using the gold-foil experiment.
Suggested a new theory base on his results.

16. Rutherford’s Gold Foil Experiment

17. Summary of Gold-Foil Experiment
A narrow beam of alpha particles was directed at a very thin sheet of gold foil (1 atom thickness).
According to the previous theory, the particles should have passed thru.
Most particles passed through and some of them bounced back or were deflected.

18. Rutherford’s Conclusions
Atoms are made up of mostly empty space.
Atoms have a positive center called a nucleus.
Most of the mass of the atom is located in it’s nucleus.
Improvement over the Plum-Pudding Model, but still incomplete.
Plum Pudding Model
Rutherford’s New Atom

19. Niels Bohr (1885-1962) Danish Physicist
Bohr debating Quantum Theory with Albert Einstein

20. Bohr Model of the Atom Planetary Model was correct.
Electrons were in orbits around the positively charged nucleus.
Based on Quantum Physics.

21. Electron Cloud Model
Electrons are not in perfect orbits around the nucleus.
Imagine, as the electron moves it leaves a trace of where it was.
This collection of traces quickly begins to resemble a cloud. 

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