Day 7: Modeling Hydrocarbons with Lewis Representations
Seeing the Invisible: Why Chemists Draw Molecules
Pick up a bottle of propane. What do you see?
However, what you don’t see is far more interesting: carbon atoms bonded to hydrogen, arranged in space, vibrating and rotating. This is the world of molecules. And it’s hidden. Chemists, of course, cannot see molecules directly. But they can model them.
In earlier lessons, we used molecular orbital (MO) theory to explain bonding in small molecules like O2 and N2. MO theory is powerful: it shows how atomic orbitals combine to form molecular orbitals, and it helps explain things like bond order, magnetism, and the energetic stability of molecules. But as soon as you add a third or fourth atom, MO diagrams become harder to construct—and even harder to interpret.
So chemists use other models too. When molecules grow larger, we often need tools that balance conceptual clarity with practicality—models that are easy to apply and reason with, even if they are not grounded in quantum mechanics. Lewis structures, for instance, are not quantum mechanical models, but they are still useful representations—a kind of chemically meaningful shorthand for valence electrons and the connectivity between atoms that gives rise to molecules.
That’s where this day begins. We will use simple molecules called hydrocarbons to explore how structure relates to behavior. Along the way, you will learn to use Lewis structures to translate between three-dimensional shapes and two-dimensional drawings. These tools help us visualize what molecules look like, how electrons are arranged, and why some species are more stable—or more reactive—than others. Even when MO theory fades into the background, these representational tools remain central to how chemists think.
Here are links to all sections of the work for Day Seven. Be sure to complete them before your whole-class meeting.
D7.1 Hydrocarbons: Why Use Lewis Structures?
D7.2 When One “Bond” is not Enough
D7.3 Substitution Patterns and Structural Variation
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