“More than fifty million unique chemical substances have been identified and the number is growing at a rapid pace. Essentially all of these are composed of the 92 naturally-occurring elements and the vast majority are from twenty or fewer of the most abundant elements.” 3.1: Elements, Atoms, and Atomic Theory 3.2: Hydrogen - The Simplest Atom 3.3: Helium - The First Noble Gas 3.4: Lithium, The First Metal 3.5: The Second Period of the Periodic Table 3.6: The Magic Octet of 8 Outer-Shell Electrons 3.7: Completing the 20-Element Periodic Table 3.8: The Brief Periodic Table is Complete 3.E: The Elements - Basic Building Blocks of Green Chemicals (Exercises) 3.R: References Template:HideTOC