This lecture begins by applying the united-atom "plum-pudding" view of molecular orbitals, introduced in the previous lecture, to more complex molecules. It then introduces the more utilitarian concept of localized pairwise bonding between atoms. Formulating an atom-pair molecular orbital as the sum of atomic orbitals creates an electron difference density through the cross product that enters upon squaring a sum. This "overlap" term is the key to bonding. The hydrogen molecule is used to illustrate how close a simple sum of atomic orbitals comes to matching reality, especially when the atomic orbitals are allowed to hybridize. Problem sets/Reading assignment: Reading assignments, problem sets, PowerPoint presentations, and other resources for this lecture can be accessed from Professor McBride’s on-campus course website, which was developed for his Fall 2008 students. Please see Resources section below. Resources: Professor McBride's web resources for CHEM 125 (Fall 2008)