1. The Changing Model of the Atom

2. 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

3. Democritus 370 – 460 BCE

4. Democitus 460 – 370 BCE Democritus believed matter was made up of “atoms” (he came up with this word) Democritus thought that each element has its own kind of atom, different in shape and size from the atoms of another element, and that all atoms move around in space.

5. John Dalton

6. John Dalton 1850 Dalton stated that all matter is made of indivisible and indestructible atoms, which differ from element to element. (Basically he agreed with what Democritus had said 2000 years ago.) He also said atoms of different elements have different properties Different elements combine in specific ways to form new substances. The billiard ball theory

7. J. J. Thompson Cathode ray tube

8. 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. 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.

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

10. Ernest Rutherford

11. Rutherford fired tiny alpha particles at a very thin piece of gold foil. He found that most of the alpha particles passed right through the gold foil as he expected However, a small number of alpha particles were deflected (as if they had bumped up against something) and some even bounced straight back like a tennis ball hitting a wall.

12. Rutherford’s gold foil experiment

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

14. Rutherford’s planetary model

15. 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. 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.

16. James Chadwick The three isotopes of hydrogen

17. Neils Bohr

18. Bohr’s Model 1913 Bohr was studying electrons in atoms (particularly hydrogen) and proposed the idea that electrons travel in discrete orbits or shells around the atom's nucleus. Each of these orbits has a certain energy associated with it (each orbit is quantized) and electrons cannot more to other orbits unless they are given the correct amount of energy. We now know his theory is not entirely correct but it is still useful today.

19. Bohr’s model

20. The Quantum Mechanical Model This is the correct model used today. 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 3D cloud that has a particular energy

21. http://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.htmlhttp://chemmovies.unl.edu/ChemAnime/R UTHERFD/RUTHERFD.html