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Featured Golden Rules of Chemistry: 5. Delocalization of charge over a larger area is stabilizing. , 7. Delocalization of pi electron density over a larger area is stabilizing.

1. Although it might not be obvious, molecular orbital theory and valence bond theory make very similar overall predictions concerning the elcctron density in the sigma bonds of molecules. When all the molecular orbitals are summed, they predict electron density existing primarily shared between the atoms (bonding) just like the valence bond overlap model does. Valence bond theory just makes more sense to humans.

2. Line angle drawings (i.e. line structures) are a useful shorthand form of structure drawing in which lines represent bonds, the joining of two lines represents a carbon atom, all H atoms attached to C are assumed but not shown, and all N,O,Cl, etc. atoms are labeled. Note: H atoms on O and N are always drawn.

3. You need to be able to think about all sigma bonding in molecules as being derived from the overlap of hybridized orbitals and/or H1s orbitals and all pi bonding as being derived from overlap of unhybridized 2p orbitals.

4. Watch this video to make sure you understand what we want!

5. Take home: According to molecular orbital theory and corraborated by many, many experiments, pi electron density (in other words pi bonding molecular orbitals) can extend over more than just two atoms. These so-called "pi-ways" are formed when 2p orbitals on three or more adjacent atoms overlap.

6. Details: When three adjacent 2p orbitals combine, three new pi molecular orbitals are created, including the lowest energy bonding "pi-way" orbital, an intermediate level orbital with electron density only on the terminal two atoms, and a highest energy antibonding orbital with nodes between each atom. This pattern is the same for all three-atom systems described by contributing structures, and for each of these sytems that are neutral or negatively-charged, there are 4 pi electrons, meaning the "pi-way" orbital and non-bonding orbitals are filled.

7. Especially for charged molecules, it is stabilizing to have pi electron density as well as charges delocalized over more than two atoms. This concept is usually referred to as "stabilization due to delocalization" or simply "resonance stabilization". Yay delocalization!!!

8. Three (or more) atom "pi-ways" are the situation resonance contributing structures are usually trying to describe. Individual Lewis structures can only describe pi bonding between two atoms, not three or more, explaining why the contributing structures are needed for these situations.

9. For pi bonding and therefore pi delocalization to occur over more then two atoms (i.e. pi-ways), parallel 2p orbitals are needed on ALL of the adjacent atoms involved, explaining why ALL of these atoms must be sp2 (or sp hybridized) and why these sytems are planar.

10. Hybridization state identification:

sp3 hybridized - only 4 sigma bonds and/or lone pairs;

sp2 hybridized - 3 sigma bonds and/or lone pairs and 1 pi bond;

sp hybridized - 2 sigma bonds and/or lone pairs and 2 pi bonds.

sp2 (or rarely sp) hybridized - all atoms having a pi bond in ANY important contributing structure (even if that atom has three sigma bonds and a lone pair on the other contributing structures!).

HOMEWORK:

Read: Sections 2.3 in the ebook textbook. This text is part of the Longhorn Textbook access program.

Take the Daily Quiz 4 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 3% of your final grade.

Finish working on the Homework Problem Set 2, due at 10 PM on Tuesday, September 10. Click here to access the Homework Problem Set 2. Note there are Aktiv Learning and Gradescope Questions, and you MUST DO BOTH. Collectively, homeworks count for 10% of your final course grade. The Aktiv Learning homework provides multiple attempts and provides feedback. It is intended to help you prepare for the Gradescope Questions, so we recommend you do the Aktiv Learning questions first.

These quizzes and homeworks are designed to keep you up to date with the course material and prepare you for the format of the exams. Taken together, your cumulative daily quiz and homework score will count for 13% of your final grade.