VSEPR

What is VSEPR?

VSEPR (valence shell electron pair repulsion) is a theory for modeling 3-dimensional molecular structures based on the placement of a central atom, ligands (atoms that attach to the central atom), and lone electron pairs.

First Steps

When identifying a VSEPR model, start with a Lewis Dot structure of the molecule. To learn more about this step, click the button below. Next, identify the formula for the shape of the molecule. The central atom is represented by an "A," the ligands are represented by an "X," and lone electron pairs are represented by an "E." More than one ligand or electron pair is represented by a subscript number after the "X" or "E," respectively.

Finding the Final Structure

Match the formula you wrote with a structure in the following image. You may need to memorize this information. Then, follow the matched formula to obtain all of the information that goes with a structure.

Lewis Dot Structures

What is a Lewis Dot Structure?

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A Lewis Dot Structure is a way of depicting the valence shell electrons  present in a molecule.

Step 1

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You will be given a molecule. For example, let's choose ammonia, which is notated as NH3.

Step 2

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Above hydrogen (H), it says "IA" underneath where it says "1." Above nitrogen (N), it says "VA" below "15." These numbers before the "As" will come in handy soon. [Image courtesy of Newsweek.com]

Step 3

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These numbers referred to in the previous step are the valence shell electrons (VSEs). Essentially, VSEs are the number of electrons in a molecules outermost shell. Hydrogen has one VSE per atom, and nitrogen has five VSEs per atom. These VSEs are represented by dots in the Lewis Dot Structure; above, you will see a visual depiction of these elements' VSEs. Since the example molecule is NH3, there are three hydrogen atoms and one nitrogen atom.

Step 4

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Next, you must combine these into the final Lewis Dot Structure. When two valence electrons are combined, they form a bond, visually shown by a dash. 

Conclusion

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In the final structure, the nitrogen has two "extra" electrons. This is called a "lone pair," and they do not take part in a covalent bond. In VSEPR notation, this molecule is AX3E.

Polyatomic Ions

Polyatomic ions are molecules that have certain charge. Of the many polyatomic ions, there are some that you should memorize. To the right, you will find the ones that should be memorized.

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