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Chemistry 11 Models of the Atom Atomic Theory Overview •  ~ 10 classes •  4 Quizzes (Oct 31st) •  2 Hand In Assignments •  1 Unit Test (~Nov 22nd) Which one is the correct representa7on of an atom? Timeline of Atomic Models •  500 BCE – 400 BCE •  Democritus: Developed the idea of atom (from the word atomos, meaning indivisible) •  He thought different “things” were made of atoms that have different shapes Compe7ng Theory •  Aristotle (a famous philosopher) proposed that matter was made of different amounts of these elements: earth, water, Rire, air •  People believed Aristotle because he was more respected and thus Democritus’ idea was ignored for many years… L Timeline of Atomic Models • Aristotle’s idea lasted for ~2000 years then …! • John Dalton came around in 1800’s… Dalton’s Model of the Atom •  John Dalton (1800’s) performed experiments that led to the acceptance of the idea of atoms. •  Matter consists of indivisible atoms. Dalton’s Model of the Atom •  Elements are made up of very small particles called atoms. •  An element is made up of a group of identical atoms. •  A particular combination of speciRic types of atoms creates a chemical compound. •  Chemical reactions involve the re-­‐shufRling of the atoms in a compound to make new compounds. Dalton’s Model of the Atom Thomson’s Model of the Atom •  In 1897, Thomson showed that atoms contained even smaller particles: electrons. Thomson’s Model of the Atom •  Atoms contain electrons. •  Atom consists of a ball of positive charge with negative charges distributed throughout the ball. •  “Plum Pudding” Model Rutherford’s Model •  In 1908, Ernest Rutherford proved Thomson wrong! Rutherford’s Model •  Rutherford performed the gold foil experiment. Rutherford’s Model Expected and Real Outcome Expected Outcome •  Alpha particles are positively charged and are 8000x heavier than electrons. •  Alpha particles would “cut” through atoms if plum pudding model was true. Real Outcome •  Most alpha particles traveled through as expected. •  Some bounced back… •  There has to be something big enough that caused the alpha particles to bounce back… Rutherford’s Model •  Most of the alpha particles (positively charged) went straight through but some bounced back. à Atom is mostly empty space with a positively charged nucleus. à Nucleus contains almost all the mass of the atom and consists of protons and neutrons. à The number of electrons surrounding the nucleus equals the number of protons in the nucleus, but electrons are much lighter than protons. Rutherford’s Model of Atom Rutherford’s Model •  Rutherford discovers protons in the nucleus. •  In 1932, James Chadwick discovers neutrons in the nucleus.