Molecular Shapes and Hybrid Orbitals

Why is molecular shape important? The shape of a molecule contributes to its properties and behavior Changing the shape can change how the molecule functions Extremely important in biological processes: Enzymes Cancer-treatment drugs

What accounts for the shape of a molecule? Valence-Shell Electron-Pair Repulsion Theory (VSEPR Theory), pronounced “vesper”: atoms align themselves to maximize the distance between the valence electron pairs https://jahschem.wikispaces.com/VSEPR

Shapes you need to know… Start with… Example # atoms bonded to central atom # lone pairs on central atom Molecular Shape Molecular Polarity linear F2 CO2 trigonal planar BF3 NO2– tetrahedral CH4 NH3 H2O trigonal bipyramidal PF5

Molecular Polarity Not the same thing as bond polarity With molecular polarity, it’s all about symmetry and whether the molecule has a dipole moment.

Molecular Polarity Two nonpolar molecule requirements: 1. Identical atoms bonded to central atom 2. No lone pairs of electrons on central atom

Shapes you need to know… Start with… Example # atoms bonded to central atom # lone pairs on central atom Molecular Shape Molecular Polarity linear F2 CO2 trigonal planar BF3 NO2– tetrahedral CH4 NH3 H2O trigonal bipyramidal PF5

Why do atoms bond? Diagram from Pearson Education 2014.

Bonding often brings atoms to a lower energy state Why do atoms bond? Bonding often brings atoms to a lower energy state Diagram from Pearson Education 2014.

Hybrid Orbitals Atoms share electrons so they can fill their outermost s & p orbitals Orbitals overlap to form a bond

Hybrid Orbitals – Example #1 Draw the orbital notation diagram for carbon. Carbon’s “standard” electron configuration: Two half-filled p orbitals... so how does carbon form four bonds? Hybridization: a theory that helps explain the shapes of some molecules

Hybrid Orbitals – Example #1 Carbon hybridizes its valence s and p orbitals Move an electron from the 2s to the 2p orbital We call these sp3 hybrid orbitals (1 s orbital and 3 p orbitals mix)

Hybrid Orbitals sp3 hybridization leads to a tetrahedral shape (bond angles 109.5°)

Hybrid Orbitals – Example #2 Draw the orbital notation diagram for boron.

Hybrid Orbitals – Example #2 Boron hybridizes its valence s and p orbitals Move an electron from the 2s to the 2p orbital We call these sp2 hybrid orbitals (1 s orbital and 2 p orbitals mix)

Hybrid Orbitals sp2 hybridization leads to a trigonal planar shape (bond angles 120°)

Hybrid Orbitals – Example #3 Draw the orbital notation diagram for beryllium.

Hybrid Orbitals – Example #3 Beryllium hybridizes its valence s and p orbitals Move an electron from the 2s to the 2p orbital We call these sp hybrid orbitals (1 s orbital and 1 p orbital mix)

Hybrid Orbitals sp hybridization leads to a linear shape (180° bond angle)

Bond Strength and Length

Sigma and Pi Bonds

Shape #1 – Linear F2 Lewis structure CO2 Lewis structure

Shape #2 – Trigonal Planar BF3 Lewis structure: NO2– Lewis structure:

Shape #3 – Tetrahedral CH4 Lewis structure: NH3 Lewis structure: H2O Lewis dot structure: