The VSEPR Model and Molecular Geometry

Greetings,

Now, we'll get into one of my favorite subjects, Structures of Molecules !

The Valence Shell Electron Pair Repulsion (VSEPR) Model

"VSEPR" is the phenomenon whereby pairs of electrons (from the Lewis Model), around the central atom of a molecule, move away from each other to positions resulting in minimum electrostatic repulsions between them.  You can remember VSEPR by saying, "Va-SEP-eR"!  Imagine two children chasing each other around (in the same direction) a tree.  At any moment, they are likely opposite of each other around the tree: Two pairs of electrons are also opposite to each other; around an atom.  Several electron-pair arrangements and their names are listed below.

 All possible scenarios are listed for the linear and trigonal planar geometries.  The tetrahedral structure name is based on "tetrahedron", which is a three sided pyramid with identical dimensions on every face (included the bottom). The 3-dimensional tetrahedral shape is indicated using an electron cloud for each pair of electrons.

Molecular Structures

Actual molecular geometries are based on electron arrangement and the number of atoms surrounding the central atom.  The tetrahedral arrangement of electrons results in three molecular geometries; bent, trigonal, and tetrahedral.  Examples of molecular geometries are shown in the figure below.

Note the lone, nonbonding pairs of electrons on the water and ammonia central atoms.  Lone pairs of electrons occupy more space than bonding electrons, therefore they push bonding electrons away.  This has the effect of decreasing the bond angle.  The small bond angle of water contributes to its "unusual" properties, compared to similar compounds.

That's all for this post.  My next post will feature covalent compound properties as they relate to molecular geometries!

Have a good one!

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