1. The Evolution of the Model of the Atom 2009-2010

2. Democritus: Greek Philosopher 460-370 B.C. 460-370 B.C. Believed that atoms were indivisible and indestructible Believed that atoms were indivisible and indestructible Did not explain chemical behavior Did not explain chemical behavior Lacked support because his claims weren’t based on the scientific method. Lacked support because his claims weren’t based on the scientific method.

3. John Dalton 1803 John Dalton pictures atoms as tiny, indestructible particles, with no internal structure. Dalton’s Atomic Theory 1.All matter is made of atoms 2.Atoms of the same element are identical 3.Atoms of different elements can mix together to form compounds 4.Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

4. JJ Thompson 1897: discovers the electron. This later leads to his "plum pudding“ model. He pictures electrons embedded in a sphere of positive electrical charge.

5. Ernst Rutherford 1911: finds that an atom has a small, dense, positively charged nucleus. Electrons move around the nucleus. Animation 1 Animation 2

6. Robert Millikin 1911: Determined the charge of an electron 1911: Determined the charge of an electron Animation of experiment Animation of experiment Animation of experiment Animation of experiment

7. Neils Bohr 1913 : the electrons move in circular orbits at fixed distances from the nucleus. An electron can gain or lose energy by changing its orbit.

8. Erwin Schrodinger 1926 develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model. The electron cloud is a visual model of the probable locations of electrons in an atom. The probability of finding an electron is higher in the denser regions of the cloud.