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A Brief History of the Atomic Theory 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 an “atom” from the Greek word atomos, meaning “indivisible” 1807: John Dalton British chemist First modern scientist to propose the existence of atoms Described it as an invisible, indestructible, solid sphere 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 1911: Ernest Rutherford British physicist Proved that atoms are mostly empty space Discovered the nucleus, which contains positively charged particles First to propose electrons circle nucleus 1913: Niels Bohr Danish scientist Proposed electrons move in different orbits, or energy levels 1932: James Chadwick British physicist Discovered the neutron Explained why atoms were heavier than the total mass of protons and electrons Current Model Based on Bohr model Electrons orbit nucleus in random paths, or electron clouds