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The Greek Model Democritus - Greek philosopher around the year 400 BC. Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually, the smallest piece of matter would be found. He used the word atomos to describe the smallest possible piece of matter. The Dalton Model Modern Electron-Cloud Model English chemist that proposed first atomic theory in 1803. Points of Dalton's Theory: All elements are composed of indivisible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by joining atoms of two or more elements. Dalton’s model looks like a marble. The Thomson Model English scientist who discovered electrons in 1897. Sometimes called the "plum pudding" model, Thomson thought of an atom as being composed of a positively charged material with the negatively charged electrons scattered through it. Looks like a chocolate chip cookie. The Rutherford Model British physicist who discovered the nucleus in 1908. Rutherford's model proposed that an atom is mostly empty space. There is a small, positive nucleus with the negative electrons scattered around the outside edge. The Bohr Model Danish scientist who proposed the Planetary Model in 1913. Electrons move in definite orbits around the nucleus, like planets moving around the nucleus. Bohr proposed that each electron moves in a specific energy level.