1. The Changing Model of the Atom
From Democritus to Bohr

3. Democritus 370 – 460 BCE

4. Democritus 460 – 370 BCE
Democritus described atoms as being indestructible.
Democritus thought that there are many different kinds of atoms, each distinct in shape and size and that all atoms move around in space.

5. Aristotle 400 BCE claimed that there was no smallest part of matter
different substances were made up of different proportions of fire, air, earth, and water.
The four element theory

6. John Dalton

7. John Dalton 1850
Dalton stated that all matter is made of indivisible and indestructible atoms, which differ from element to element.
The billiard ball theory

8. J. J. Thompson
Cathode ray tube

9. J.J. Thompson 1897
For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen
Thomson found that the glowing stream would bend toward a positively charged electric plate.

10. J.J. Thompson 1897
Thomson theorized, that the stream was made up of small particles, pieces of atoms that carried a negative charge.
These particles were later named electrons.

11. Thompson’s Plum Pudding Theory Or Rasin Bun Theory

12. Ernest Rutherford

13. Ernest Rutherford
Rutherford fired tiny alpha particles at solid objects such as gold foil.
He found that most of the alpha particles passed right through the gold foil

14. Ernest Rutherford
A small number of alpha particles passed through at an angle (as if they had bumped up against something)
and some bounced straight back like a tennis ball hitting a wall.

15. Rutherford’s Planetary model
In 1911, Rutherford proposed a revolutionary view of the atom.
He suggested that the atom consisted of a small, dense core of positively charged particles in the center (or nucleus) of the atom and mostly empty space.

16. Rutherford’s Planetary model
Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the center and the electrons revolving around the nucleus.

17. Rutherford’s planetary model

18. James Chadwick 1932
Chadwick discovered a third type of subatomic particle, which he named the neutron.
Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus.

19. James Chadwick 1932
Neutrons always reside in the nucleus of atoms and they are about the same size as protons.
Neutrons do not have any electrical charge; they are electrically neutral.

20. James Chadwick

21. Neils Bohr

22. Bohr’s Model 1913
The Bohr model of the atom says that electrons travel in discrete orbits around the atom's nucleus.
His theory was radical and unacceptable to most physicists at the time

23. Bohr’s model

24. The Electron Cloud Model
The electron cloud model uses the basic idea of Bohr’s model except that the electrons are not found in distinct orbits but their position can be thought of as in a cloud that has a particular energy

25. http://videos. howstuffworks. com/science/atoms-videos-playlist