Atomic history Democritus first person to use the term atom in 400 B.C. Democritus defined the atom as being indivisible Conservation of mass: mass cannot be created or destroyed during chemical and physical changes.

Conservation of Mass + Before reaction After reaction 0 g H2 40 g O2 High voltage electrodes Before reaction glass chamber High voltage After reaction 0 g H2 40 g O2 + 85 g total O2 H2O H2 5.0 g H2 O2 “Conservation of Mass” (Lavoisier)   Description: This slide illustrates a reaction between hydrogen and oxygen in a nonstoichiometric mixture of these gases. Basic Concepts ·         Mass and atoms are conserved in chemical reactions. ·         When non-stoichiometric quantities of substances are mixed, they react in stoichiometric proportions. Any reactants in excess remain unreacted. Teaching Suggestions Explain that the first diagram shows the amount of oxygen and hydrogen in a closed chamber. A spark passes between the electrodes, causing the O2 and H2 to react rapidly. The second diagram shows what is in the chamber after the reaction. Use this slide to illustrate that reactants combine in the stoichiometric proportions. Stress that is is not sufficient to know the amounts of starting materials present. One must also know the amounts of reactants that will take part in the reaction. Questions What is the ratio of the mass of O2 to H2 before the reaction? What is the ratio of the number of moles of O2 to H2 before the reaction? How do you account for the fact that the mass of the chamber and its contents is the same before and after the reaction. Why is some oxygen left in the chamber after the reaction? What are the masses of H2 and O2 that take part in the reaction? What is the ratio of the mass of O2 to H2 taking part in this reaction? What is the ratio of the number of moles of O2 to H2 taking part in the reaction? Why is this mole ratio different from the mass ratio? If there were twice as much H2 in the chamber (10 g) but the same amount of O2 (80g), what would you expect to find in the chamber after the reaction? Explain your answer. + 80 g O2 45 g H2O 85 g total Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

Dalton’s Atomic Theory All matter is made up of atoms Atoms of the same element are identical and atoms of different elements are different Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in whole- number ratios to form compounds. In chemical reactions, atoms are separated, combined, and rearranged. 2. Atoms of different elements are different

Major discoveries Thomson conducted the cathode ray experiment Thomson concluded that cathode rays were composed of identical negatively charged particles Millikan concluded that electrons are present in atoms of all elements. Rutherford conducted the gold foil experiment Rutherford discovered the nucleus Goldstein discovered the proton Chadwick discovered the neutron