Section 3: Particles of Matter • Objectives: identify atoms as the smallest particles of an element describe Dalton’s theory of atoms identify chemical bonds as the force holding atoms together in molecules I. Early Ideas About Atoms FYI: This coin was in use during Democritus’s time. (440 B.C.) A. Democritus thought the smallest particle in an object like this silver coin was called an atomos. B. Dalton’s Ideas About Atoms a. b. c. C. In 1897, British scientist J.J. Thomson discovered small partic inside the atom. Thomson’s cathode-Ray tube experiment Discovered negatively charged particles called elelctrons FYI: Thomson revised atomic theory to include discovery of electrons. He know overall charge of atom was neutral, but didn’t know Location positive charged particles. Proposed possible model called Plum Pudding model. D. 1909: Rutherford’s “Shooting Gallery” Experiment Ernest Rutherford further tested Thomson’s Plum Pudding Model. Send stream of radium at gold foil. Some particles were deflected. Rutherford proposed positively charged region called nucleus. Most of atom’s mass is located here. Proposed positively charged that passed close by the nucleus were pushed away by positive nucleus. D. Bohr’s Atomic Model 1913 Niels Bohr, a Danish scientist working for Rutherford suggested electrons travel around nucleus in definite paths. Paths were located certain distances from nucleus. Electrons could jump from level to level. E. Modern Atomic Theory: Electron Cloud Model Austrian scientists Schrodinger and Heisenberg revised Bohr’s model. Stated electrons do not travel in definite path as Bohr suggested. exact path of moving electrons can not be predicted. Where electrons are likely to be found is region called electron clouds. Ideas About Atoms today The pattern that forms when a beam of electrons is directed at a sample of aluminum. Analysis shows atoms have a diameter of about 0.00000003 cm. Three hundred-millionths of a centimeter. It would take 50,000 aluminum atoms to equal thickness of sheet of foil. II. Atoms and Molecules A. force that holds atoms together is a chemical bond. a. two or more atoms bonded together is a molecule.