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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.

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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 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

Bond order correlates with the bond strength – the higher the bond order, the stronger the bond. Bond energy also increases with bond order.

Since bond energy is the energy needed to break bonds, you can understand why having more bonds between atoms will increase the attractive forces between the electrons, which increases the bond energy.The strength of the one bond order is much less than the strength of the bond order four. Think of it this way. You have two balls that are held together by strings.

If you have one string between those two balls, it isn’t hard to pull them apart. This is similar to a single bond. If you have four strings between the two balls, it is much harder to pull them apart, just like it is when you have four bonds.

Lesson Summary

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.

The molecular orbital theory helps determine bond order because it shows how many bonds the atoms in the molecules make with each other. Bond strength correlates with bond order because the more bonds between atoms, the stronger they are.

Learning Outcomes

At the end of this lesson, you will be able to:

  • Explain the molecular orbital theory
  • Describe bond order and bond strength

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