Using Orbital Hybridization and Valence Bond Theory to Predict Molecular Shape

  • 0:01 Hybridization
  • 1:37 Orbital Hybridization Theory
  • 3:00 Sigma and Pi
  • 3:24 Number of Orbitals
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

You'll learn how to explain how shapes of molecules can be predicted using valence bond theory and hybridization. When finished, you'll understand the difference between sigma and pi bonds and how the VSEPR theory, along with the hybridization theory, helps predict the shape of a molecule.

Hybridization

When you think of hybrid, you may think of a car, like the Toyota Prius, or an animal, like the zebroid or the liger. But in this lesson, we are using the term 'hybrid' to refer to the orbitals of electrons in atoms.

Hybridization is the mixing of two or more atomic orbitals to form new orbitals that describe the covalent bonding in molecules. Orbital hybridization shows the relationships between the geometry of a molecule and the orbitals of the bonding electrons. Hybridization works because the net energy of the hybridized bonding electron orbitals is reduced compared to the un-hybridized orbitals.

Valence bond theory helps us determine how many bonds there are between two atoms in a molecule
Valence Bond Theory

Orbital Hybridization Theory

In order to figure out the hybridization, we need to know the valence electrons in the participating atoms and the valance bond theory. The valence bond theory essentially says that all bonds are made by an atom donating a valence electron to another atom to complete its octet. The theory, combined with knowledge of valence electrons, tells us how many bonds there are between two atoms in a molecule. The VSEPR theory, as you've learned previously, helps predict the shape of a molecule based on the repulsion of the electrons in the orbitals. The VSEPR theory fails to explain all of the interactions scientists see in molecules, though. So they've developed the concept of orbital hybridization.

Take methane as an example (CH4). The carbon atom has four valence electrons: two in the 2S orbital and two in the 2P orbital. With what you've learned, it might not make sense that these four electrons form bonds in a tetrahedral shape, because two electrons in the S orbital are already paired up compared to the two single P orbital electrons. You may think it would only have three orbitals, not four.

You also know that the S orbital is spherical and the P orbital is dumbbell-shaped. To explain the known bonding of the carbon atom, you have to assume that the 2S and 2P orbits get combined and rearranged to make four orbitals. In other words, they get hybridized.

Hybridization can be compared to mixing different colors of water together
Hybrid Analogy

A good analogy for understanding hybridization is colored water. Start with one beaker with 50mL red water that represents the 2S orbital and three beakers, each with 50mL blue water, that represent three 2P orbitals. Mix all four beakers and get 200mL of purple water. Divide the purple water into four beakers with 50mL in each. These are the hybrid orbitals. Just as each of the beakers is made of a mixture of red and blue water, each hybrid orbital is made of a mixture of 2S and 2P orbitals.

Sigma and Pi

There are two types of covalent bonds: the very strong sigma and the not-as-strong pi. The sigma bond is when two orbitals directly overlap but there is only one bonding interaction. A pi bond is weaker than the sigma bond. This overlap occurs when two orbitals overlap and there are two bonding interactions. It looks like two dumbbells put side-by-side and overlapped.

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