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Review of VSEPR Theory

Review of VSEPR Theory

Author: Katie Smith
Description:

To review VSEPR theory.
To understand how to determine the shape or geometry of a molecule.
To connect chemical names, molecular formulas, Lewis dot structures and molecular geometry.

A series of slideshows will review VSEPR theory and work through the process of connecting your knowledge of chemical naming with molecular formulas, Lewis Dot structures and molecular geometry.

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Tutorial

The Way Things Shape Up

In everyday life, the shape of an object plays a huge role in how it functions.  You don't believe me?  Try putting your right shoe on your left foot.  The next time somebody extends their right hand to shake, reach for it with your left hand.  Put on a pair of glasses where the ends curl up, instead of down.  Or just try to drive a car with square tires. 

Shape is important at every level, from the macroscopic (think aerodynamic rocket ships) to the microscopic, and even smaller still -- at the level of each individual molecule.  The way that a molecule is shaped will affect how it interacts with other molecules, or how they "shake hands" with one another. 

To understand the shape of a molecule, we begin by considering the geometry around a single atom.  The groups surrounding that central atom will spread themselves out in a way that best minimizes electron repulsion between the groups.  This repulsion between valence electrons (the ones involved in bonding and the lone pairs) is summarized in Valence Shell Electron Pair Repulsion, or VSEPR, Theory.

Before we get started, you should be comfortable with drawing Lewis Dot Structures.  If you're not, check out this packet for a review.

A Review of VSEPR Theory

Putting It All Together

Imagine this situation:

There are only 10 minutes left in class.  You're feeling pretty good, and you've finally managed to connect

 

and understand that both of those represent something that, really, looks like

 

Right before the bell rings, you're told that your homework is to determine the shapes of carbon tetrachloride and the nitrite ion, and you begin to panic.  You've never even HEARD of carbon tetrachloride, how do you know what shape it is?!?!??!

So, What Do You Do?

First, go for a run and burn off some of that excess energy.  Once you're a bit more relaxed, remember that you can work through this step by step.

What were you given?  Well, you were given a name.  The name has a number indicator in there (tetra), and we're trying to determine a molecular geometry, so already you know it's a covalently bonded compound.  So, you pull out your notes on Naming Binary Covalent Compounds and realize that carbon tetrachloride has the molecular formula of CCl4.  We're getting somewhere.  Once you have a formula, you realize that you can create a Lewis Dot Structure for the molecule, and you write out something like this:

Suddenly, the problem doesn't look any different than the ones you did in class.   You count up the number of electron groups around the central atom (four), determine how many of them are lone pairs (none) and identify the geometry as being tetrahedral!

Piece of cake, really.  You repeat the process for the nitrite ion (remembering the name from your lesson on polyatomic ions, of course), and you're on your way to the next assignment.

Practice Problems