Introduction to Atoms Chapter 14 Section 1
History of Atom All atoms share the same basic structure During past 200 years, scientists have proposed different models
Dalton’s Model Based on experiments, Dalton developed a theory of structure of matter 4 main concepts: All matter is composed of tiny, indivisible particles called atoms Atoms of each element are exactly alike Atoms of different elements have different masses Atoms of different elements can join to form compounds
Dalton’s Model
Thomson’s Model End of 1800s Thomson discovered that atoms were not simple, solid spheres Atoms contained subatomic particles Very small, negatively charged Called them electrons
Thomson’s Model Also knew that atoms were electrically neutral Must contain enough positive charge to balance negative charge of electrons Developed model where electrons were stuck into a positively charged sphere Like chocolate chips in cookie dough
Thomson’s Model
Rutherford’s Model By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons 1911—Rutherford begins to test theory His experiments led him to believe that protons are concentrated in a small area at center of atom Called this area the nucleus
Rutherford’s Model Rutherford’s model describes an atom as mostly empty space, with a center nucleus that contains nearly all the mass Like the pit in a peach
Bohr’s Model Modified Rutherford’s model in 1913 Proposed that each electron has a certain amount of energy Helped electron move around nucleus Electrons move around nucleus in region called energy levels Energy levels surround nucleus in rings, like layers of onion
Bohr’s Model Has been called planetary model Energy levels occupied by electrons are like orbits of planets at different distances from the sun (nucleus)
Electron Cloud Model Model accepted today Electrons dart around in an energy level Rapid, random motion creates a “cloud” of negative charge around nucleus Electron cloud gives atom its size and shape
Electron Cloud Model