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Theories of the Atom Early Greek Theories Democritus (around 400 BC) No experimental evidence. Proposed that: matter is made of indivisible particles called _____________ Aristotle (around 350 BC): supported the idea of _________ ____________ Aristotle's ideas lasted for almost 2000 years John Dalton (1807) Dalton's atomic theory All matter is composed of ____________, ________________ particles called atoms All atoms of an ____________ have _______________ properties Atoms are ___________________ to form new substances in chemical reactions, but are never _______________ or ________________ J.J. Thompson (1897) Used a cathode ray tube and discovered atoms contain ____________________ charged particles He called these "_______________ " and his model of the atom was known as the "raisins in a bun model" The electrons were considered embedded in a _______________ "dough" Ernest Rutherford (1909) The Gold Foil Experiment Most of the positive particles went through the foil but small percentages were deflected at large angles. Rutherford suggested that: the center of the atom (called the ____________) has a _______________ charge and contains most of the atom's ____________ the nucleus is surrounded by a cloud of _____________ charged ______________ most of the atom is ____________ ____________ James Chadwick (1932) discovered undetected ____________ particles in the atom called ____________ neutrons have the ____________ mass as protons Niels Bohr (1913) From experimental evidence: electrons orbit the nucleus (like planets around the sun) each orbit holds a certain maximum of electrons, _____ in the first and _____ in each of the second and third orbits This model of the atom is called the Bohr-Rutherford model Carbon atom