Castles in the Air

As with friends, families, and communities, the truly interesting behavior starts once bonding has occurred and the structure is in place. The relationship dynamic for electrons is called “Valence Shell Electron Pair Repulsion”, which is a term that will never be hashtagged ever. VSEPR means that once electrons have bonded to form pairs, they move as far apart as possible from other electron pairs. This gives rise to many different structures as the pairs jockey for space. It’s important to remember that these structures are three-dimensional even though we draw them with dots, arrows and equations.

For example, the molecule SCL4 has five electron pairs around the S atom which creates a Trigonal Bipyramid like this:


The molecule XeF4 has six electron pairs around the Xe atom which creates an Octahedral structure that looks a lot like the jacks that we used to play with:


I learned 10 structures this week, ranging from the lowly Linear, Bent, and T-Shape all the way to the magnificent Tetrahedral and Square Pyramidal. I also learned that these bonds build more than just beautiful structures. The nature of the bond has a direct effect on the physical properties of the compound. Strong bonds cause high melting points, high boiling points, and low vapor pressures. I wonder what new materials are out there waiting to be created with just the right combination of structure and strength?

Tomorrow I am travelling to attend a conference about learning. I hope to eventually build just the right classroom combination, even though right now the areas of teaching and learning seem more like alchemy – all mystery and magic – and less like the balanced and predictable study of electrons that I find so comforting.

Next Up: Name and Number, Please

A Quantum Leap

The autonomy of on-line learning is intoxicating and disorienting. I was irritated when I could not get an answer to a simple question this week from my pricey certification program: “How was the value of v used in the equation example on page 5 of Chapter 2.4 generated?” Later that same morning I was overwhelmed by the gift of Salman Kahn delivering the best Chemistry lecture I have ever heard in my life at his site that promises to be “Completely Free, Forever.” This landscape of learning gets curiouser and curiouser every day; a Wonderland not unlike the world of Chemistry in the early 1900’s.

Thanks to classic physics, we were mostly comfortable with our models of matter (particles) and light (waves).  However, there were four behaviors that consistently did not fit and we knew that if we didn’t understand all of it then we didn’t understand any of it. So Planck proposed that in some situations, solids act like waves. Even though his math checked out, this was at such odds with accepted theory that even he did not accept his findings. Then Einstein used Planck’s model, and his constant, and applied it successfully to light waves. Black Body Radiation, the Photoelectric Effect, Absorption and Emission of Light, and Atomic Structure and Stability were solved! The Theory of Quantum Mechanics was born, where light is made of particles and solids are in constant vibration. I am still a little disoriented but I am looking forward to taking a closer look next week.

Next Up: Through the Looking Glass