* What does that mean? * Theory: Explanation * Atomic: Of atoms * How did we develop this theory? * Began with the Greeks * Democritus- 470-380 B.C. * Matter cannot be broken down indefinitely. At some point you end up with a piece that cannot be divided. That smallest piece he called an atom, from the Greek word atomos, which means “indivisible” * All matter is composed of atoms, which are bits of matter too small to be seen. These atoms cannot be further split into smaller proportions. * There is a void, which is empty space between atoms. * Atoms are completely solid. * Atoms are homogeneous, with no internal structure. Atoms are different in size, shape and weight. * Democritus’ theory held until John Dalton * Leucippus * Aristotle * John Dalton * English chemist * “Father of the modern atomic theory” * All elements are composed of atoms, which are indivisible and indestructible particles. * All atoms of the same element are exactly alike; in particular, they have the same mass. * All atoms of different elements are different; in particular, they have a different mass. * Two or more elements are joined together to form a compound. * Atoms are joined together in definite whole #ratio’s * “Law of Definite Proportions” * J.J. Thomson * 1897 * Found that cathode rays could be deflected electric field * Showed that cathode “rays” were actually particles * Electron: Particles given off by the cathode, fundamental unit of negative space * Atoms are not just solid spheres and are mostly empty space * Smaller subatomic particles called electron * Electrons are much lighter than