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Development of the Atomic Theory Chapter 6 Section 1 The Beginning of Atomic Theory • An atom is the smallest particle an element can be divided into and still have the properties of that element. Ancient Greece • Around 440 B.C., Democritus proposed that matter is made up of “uncuttable” particles which are always moving. • He called the particle an atom. Dalton’s Atomic Theory • All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. • Atoms of the same elements are exactly alike, but different from other elements. • Atoms join with other atoms to make new substances. Thomson’s Discovery of Electrons • He discovered there are small particles inside the atom and that atoms can be divided. • He discovered that some particles had a negative charge called electrons. • He didn’t know how they were arranged, but thought they might look like chocolate-chip ice cream. Cathode Ray Tube Rutherford’s Atomic “Shooting Gallery” • He aimed a beam of positively charged particles into gold foil. • Most passed through, but some bounced back. The Nucleus & the Electrons • Rutherford discovered that atoms are mostly empty space. • He said atoms have a tightly packed center called a nucleus. • He developed a new model of the atom. • He studied the way atoms react to light. • He suggested that electrons travel around the nucleus in definite paths. Bohr’s Electron Levels The Modern Atomic Theory • Schrodinger and Heisenberg showed that electrons do not travel in definite paths. • The exact path can not be predicted. ENERGY LEVELS • Each electron cloud exists at a certain energy level. • The energy of each electron keeps it in motion around the nucleus of the atom. SILVER The Size of an Atom • Atoms are very small, yet the atom was discovered without seeing a single atom. • There are 20 thousand billion, billion atoms in a penny. OBSERVING ATOMS • Atoms are so small that light waves are too large to be used to observe them. • The tools that scientist now use to observe atoms include the electron microscope. End of Section 1 Electron Microscope image of Silicon Nitrate Video