Molecular Orbital Theory
Further study: an original comparison of bonding models; molecular-orbital theory does not become a required geometry lesson.
Before this connection: Lewis diagrams, molecular geometry and sigma/pi bonding.
Beyond AP Physics 1
Original enrichment beyond the AP requirements; relevant foundational ideas are linked separately.
Chemistry I · original Chapter 5 · combined Chapter 5
Section Breakdown
- Picturing Orbital Hybridization
- Comparing VSEPR to MO theory
Section 4
Molecular Orbital Theory
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3-D 5.4.1 Molecular Orbital representation of CH4
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Download original COLLADA modelThe prediction of the path as a function of the nuclei in the molecule is actually done by the combining of the orbitals of all involved molecules.
Let’s look at how the MO theory would represent Methane:
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3-D 5.4.2 Ethenol as shown by the MO model
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3-D 5.4.3 Partial Structure of Ethenol
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Download original COLLADA modelHere is the MO representation of Ethenol, CH2CHOH, which shows sp2 hybridization on the C atoms, and sp3 hybridization on the O atom:
Now let’s compare that to the VSEPR model.
To do the VSEPR model, you need to pick a central atom. We first pick Carbon. Using the AXE method, we find that Carbon has an AXE number of AX3E0. This gives us a trigonal planar structure. We’ll start by showing what we have so far:
Now, we still have to add the atoms to that end Carbon.
Setting Carbon as the central atom, we see that it is connected to the Hydrogen, Oxygen, and the first Carbon, giving it an AXE number of AX3E0, another trigonal planar set-up.
Let’s go ahead and add that.
Now we add the final hydrogen, and get the final structure, which looks not unlike the MO theory predicted. The final structure is:
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3-D 5.4.5 Final Structure of Ethenol
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3-D 5.4.4 Partial Structure of Ethenol
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Download original COLLADA modelThe advantages of Molecular Orbital theory are best seen in larger molecules, so that goes beyond the scope of this textbook.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Orbital hybridization
Hybridization is a localized bonding model that combines atomic-orbital descriptions on an atom. It can help describe bond directions but is not a statement that isolated atomic orbitals physically mix as a timed step. For example, an sp³ model describes four tetrahedrally directed orbitals around carbon in methane.
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VSEPR
VSEPR is a model for predicting molecular geometry from electron domains around a central atom. Bonding domains and lone pairs arrange to reduce repulsion, and a multiple bond counts as one domain for the basic geometry classification. For example, water has four electron domains around oxygen but a bent molecular shape.
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Molecular orbital
A molecular orbital is an electronic state described over a molecule rather than assigned exclusively to a single atom. Molecular-orbital models can describe bonding, antibonding, and nonbonding electron arrangements. For example, occupying a bonding orbital between two atoms can contribute to holding those atoms together.
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Atomic orbital
An atomic orbital is a quantum state used to describe an electron in an atom. Its spatial description gives a probability distribution rather than a definite path around the nucleus. For example, an s orbital has a spherical spatial probability distribution, and one orbital can accommodate at most two electrons with opposite spins.
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Valence electron
Valence electrons are the electrons most directly involved in an atom's bonding and chemical behavior. For main-group atoms, they occupy the highest occupied principal shell. For example, neutral chlorine has seven valence electrons, helping explain its tendency to form a chloride ion.
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Electron delocalization
Electron delocalization means that an electronic description extends across multiple atoms rather than being confined to one localized bond or atom. It can affect bond equivalence and charge distribution. For example, nitrate's bonding electrons are described across several nitrogen–oxygen positions.