Our understanding of atoms, molecules, and chemical reactivity are based on models. Lewis structures combined with Valence Bond Theory are some of the first models you learned as a chemistry student. However, it turns out they fail in a lot of important circumstances; generally they are outdated, but useful for teaching basic chemistry. Modern understanding of atoms, molecules, and chemical reactivity is based on quantum mechanics. This section will review how we got to quantum theory, and how everything we know is based on a mathematical equation that explains the hydrogen atom. Later we will discuss how combination of the mathematical functions for individual atoms can be combined to extend quantum theory to describe molecules and bonding. Spoiler alert: what you'll learn is that there is no such thing as a localized electron pair (lone pairs and bonded pairs on or between atoms do not exist!).