By Víctor Ontiveros


What is the VSEPR theory?

First things first, this isn't English, and for it to sound English it's pronounced "vesper". It stands for "valence shell electron pair repulsion." It's main purpose is to predict the shape of the molecule with its linear geometry and molecular geometry.

The main idea of VSEPR theory is that the valence electron (pairs) surrounding an atom repel each other. They therefore create a shape that minimizes the tension and repulsion, and gives a molecule a shape.



Electronegativity Values

Each element has its own electronegativity value. The difference in the values is what shows the polarity of the molecule.

Common Atoms and their Electronegativity

Element EN
Carbon 2.5
Oxygen 3.5
Hydrogen 2.1
Phosphorus 2.1
Fluorine 4.0

Using VSEPR Theory

The molecular geometry (arrangement of atoms to minimize tension) can determine the polarity of a molecule.

Atoms Bonded with Central Atom Lone Pairs of Electrons Molecular Geometry Bond Angle(s) Polairy
2 0 Linear 180 Non-Polar
3 0 Trigonal Planar 120 Non-polar
2 1 Bent Trigonal Planar Aprox. 120 Polar
3 1 Trigonal Pyramidal Aprox. 107 Polar
4 0 Tetrahedral 109.5 Non-Polar
There are more scenarios, but these are common.

You will notice the molecular geometries that are bent are polar, this is because of uneven distribution causing the bent shape.


Carbon Dioxide (CO2)

co2 molecular geometry co2 lewis

Non-Polar: Equal Distribution

Ammonia (NH3)

nh3 molecular geometry nh3 lewis

Polar: Unequal distribution due to an unshared electron pair on Nitrogen, the central atom.

Why should I care?

As previously mentioned, VSEPR theory predicts the shape and angle of molecules. This is practical because it will show if a substance will mix with another substance based on it's lewish structure of or formula. (Like disolves like.) The picture on the left shows oil (non-polar) and water (polar) don't mix because of polarity. In a nutshell, predictions on a molecule's geometry shows how it reacts with others, thus predicting even more scenarios.

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