1. A Brief History of the Atomic Theory

2. 460 B.C. : Democritus Greek Philosopher Proposed that matter cannot be broken down indefinitely At some point you end up with a piece that cannot be divided He called the smallest piece and “atom” from the Greek word “atomos” meaning “indivisible”

3. 1807: John Dalton British chemist First modern scientist to propose the existence of atoms Described it as an invisible, indestructible, solid sphere

4. 1898: Sir J.J. Thomson British physicist Proposed “plum-pudding” model An atom is a solid mass of positively charged material with negative charges (electrons) scattered throughout Credited with discovering the electron

5. 1911: Ernest Rutherford British physicist Proved that atoms are mostly empty space Discovered the proton Discovered the nucleus, which contains positively charged particles (protons) First to propose electrons circle nucleus

6. 1913: Niels Bohr Danish scientist Proposed electrons move in different orbits, or energy levels

7. 1932-Chadwick Model British Physicist Discovered the neutron. The existence of neutrons explained why atoms were heavier than the total mass of their protons and electrons.

8. Current Model Based on Bohr model Electrons orbit nucleus in random paths, or electron clouds Often called the “cloud” model “cloud” model

9. John Dalton’s Atomic Theory of Matter All elements are composed of atoms that cannot be divided. All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses. An atom of one element cannot be changed into an atom of a different element. Atoms cannot be created or destroyed in any chemical change, only rearranged. Every compound is composed of atoms of different elements, in a specific ration.