1. Atoms- Basic Units of Matter

2. The atom Parts of an atom: – Protons: positively charged particles; located in the nucleus – Neutrons: particles with no charge; located in the nucleus – Electrons: negatively charged particles; located AROUND the nucleus in the electron cloud Regions of the atom: – Nucleus: center of the atom that is positively charged – Electron cloud: region surrounding an atomic nucleus where an electron is most likely to be found; electrons can be anywhere

3. Size of the atom As tiny as atoms are, electrons, protons and neutrons are even smaller. Electrons have only about 1/2000 the mass of protons or neutrons Visualize the nucleus as the size of an ant, the atom as a whole would be the size of the football stadium. How small is an atom?

4. The Law of Conservation of Mass The total mass before chemical reaction is the same as the total mass after reaction A chemical reaction is a process in which atoms in the starting materials rearrange to form products with different properties

5. The Law of definite proportions Any pure compound always contains the same elements in the same proportion by mass This law applies to any compound no matter where the sample comes from or how large or small it might be EXAMPLE: Water is always H2O if it comes from your sink or an ice cap on Mars

7. History Review Democritus – first person to use the word atom, comes from the Greek word atoma, means “indivisible” John Dalton came up with the atomic theory JJ Thomson did experiments using a cathode ray tube, came up with model that included positive and negative charges Ernest Rutherford did the Gold Foil experiment, which changed the model to a positive nucleus surrounded by electrons Niels Bohr determined that electrons move in specific distances from the nucleus based on how much energy they have

9. Questions – Copy Question and answer Describe the way electrons, protons, and neutrons are arranged in an atom (pg174-175) Construct diagrams to illustrate Thomson’s, Rutherford’s and Bohr’s models of the atom. (pg193)