Molecular Orbital Theory: Tutorial and Diagrams

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  • 0:05 Molecular Orbital Theory
  • 1:17 Shapes of Orbitals in…
  • 2:40 Bond Order
  • 3:14 Bond Strength
  • 3:59 Lesson Summary
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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.

Learn how to sketch the overlap of orbitals to form sigma and pi bonds. Use the molecular orbital theory to determine bond order. Discover how bond order affects bond strength and bond energy.

Molecular Orbital Theory

You have learned that atoms have electrons that reside in orbital levels, and you know how to represent the electrons as dots or dashes. This is the easy way to think about electrons in atoms, but it makes an assumption that electrons are static characters, and they're not. When we say that an electron is in the 2P orbital, we are essentially saying that the probability of it being found in that orbital is high. You won't necessarily find it there 100% of the time. You were introduced to the shapes the orbitals make in another video. But just to review, the S orbital makes a circle, or sphere; the P orbital makes a dumbbell shape in either the X, Y or Z plane. The D orbital makes a double-dumbbell or crisscross shape in the X, Y or Z plane.

So, all of these things you have learned about electrons in atoms also apply to the electrons in molecules. The molecular orbital theory describes how the orbital shapes combine when atoms combine into molecules. It makes the assumption that electrons don't belong to any individual atom, but are under the influence of the entire molecule and belong to the entire molecule as a whole.

Shape of Orbitals in Molecules

In order to figure out the shape that electron orbitals make in a molecule, you need to look at the shape of the orbitals in the atoms and how they combine and overlap constructively or destructively.

Electrons spin in their orbits, either clockwise or counterclockwise, which is sometimes referred to as positive and negative (this is different than the negative charge of an electron). If the valence electrons in a molecule are either both positive or both negative and combine, they will constructively interfere, and their orbital shapes add. If the valence electrons are negative and positive, they experience destructive interference and their orbital shapes subtract.

When two S orbitals, which are round, combine constructively, it looks like this:

S orbitals combine constructively
s orbitals combine

These are sigma bonds. If it combines destructively, it looks like this:

S orbitals combine destructively
two s orbitals combine

P orbitals are harder. The dumbbell shape can add constructively or destructively in two different ways, either side-by-side or overlapping. These are pi bonds.

A variety of shapes are possible when combining P orbitals
Combining P Orbitals Results

When both of the orbitals are in phase, either both positive or both negative, then the electrons in the bonds are at their lowest energy level and bonding takes place. What this means is that the electrons will try to be in the orbital that is the lowest energy, so they will always try to be in the constructive orbital.

Bond Order

Bond order refers to how many bonds there are between two atoms. For instance, you know that carbon makes four bonds. It can bond once with four different hydrogens, and each bond has a bond order of one. Or, the carbon can bond with itself four times for four bonds. This would be a bond order of four. You can determine bond order from counting the number of constructive interferences between the electrons in each orbital shape because these are the places where energy is least and bonds are formed.

Bond Strength

The higher the bond order, the stronger the molecular bond
Higher Bond Order More Strength

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