CHEM 1211K 1st Edition Lecture 4 Outline of Last Lecture I Chapter 1 Matter Measurement and Problem Solving Review Chapter 2 Atoms and Elements II Modern Atomic Theory A Law of Conservation of Mass by Antoine Lavoisier In a chemical reaction matter is neither created nor destroyed Total mass of reactants Total mass of products B Dalton proposed a theory of matter based on it having ultimate indivisible particles to explain these laws 1 Each element is composed of tiny indestructible particles called atoms 2 All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements the atom for each element is unique 3 Atoms combine in simple whole number ratios to form molecules of compounds 4 In a chemical reaction atoms of one element cannot change into atoms of another element III 2 4 The Discovery of the Electron 2 4 1 Electrical Charges Two kinds of charge called and Opposite charges attract attracted to Like charges repel repels repels To be neutral something must have no charge or equal amounts of opposite charges 2 4 2 Cathode Rays Glass tube containing metal electrodes from which almost all the air has been evacuated These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute When connected to a high voltage power supply a glowing area is seen emanating from the cathode IV 2 5 The Structure of the Atom A The structure of the atom contains many negatively charged electrons B These electrons are held in the atom by their attraction for a positively charged electric field within the atom there had to be a source of positive charge because the atom is neutral Thomson assumed there were no positively charged pieces because none showed up in the cathode ray experiment V Rutherford s Nuclear Model 1 The atom contains a tiny dense center called the nucleus a the amount of space taken by the nucleus is only about 1 10 trillionth the volume of the atom 2 The nucleus has essentially the entire mass of the atom the electrons weigh so little they give practically no mass to the atom 3 The nucleus is positively charged the amount of positive charge balances the negative charge of the electrons 4 The electrons are dispersed in the empty space of the atom surrounding the nucleus Current Lecture