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
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?
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
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
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