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Valence Shell Electron Pair Repulsion Theory (VSEPR) Review
A review of general chemistry 1 VSEPR theory, including electron domain geometries and molecular geometries, including expanded octets.
Brief Lewis Structure Review
A Lewis structure gives structural information about a molecule. It doesn’t give information about the overall shape of the molecule (how the atoms are arranged in space).
All the atoms in a Lewis structure are drawn in the same plane (in 2D).
Steps for drawing Lewis structures:

Sum the valence electrons from all atoms, taking into account charge (add electrons for a negative charge, +1 electron per negative charge, and subtract electrons for a positive charge, 1 electron per positive charge).

Write the symbols for the atoms, taking into account the atom connectivity, and show which atoms are attached to which. Chemical formulas are often written in the order in which atoms are connected. Often there will be a central atom which is generally the atom with the lowest electronegativity (except for hydrogen which is NEVER a central atom), and it is written first while the other atoms surround the central atom. Lastly, add lines connecting the atoms. Each line represents 2 electrons, subtract from the total valence electrons.

Complete the octets around all atoms except the central atom (subtract from the total valence electrons).

Place leftover electrons around the central atom until you no longer have valence electrons left.

Ensure that all atoms have an octet*. If they do not, move electrons from nonbonding positions to bonding positions, making multiple bonds until all atoms have octet.

Calulate the formal charge for each individual atom in the molecule using the formula: formal charge = (# of valence electrons)  (# of bonds)  (# of nonbonding electrons)

If the species is an ion, put brackets around the ion, and indicate the overall charge outside of the brackets at the top right.
*Some species usually lack an octet, such as hydrogen and helium (2 electrons), lithium and berylium (4 electrons), and boron and aluminum (6 electrons). Others can expand their octet (only elements in period 3 or below).
Molecular Shapes and Bond Angles
Unlike what we show with Lewis structures, molecules have a three dimensional, 3D, shape . We use the Lewis structures in order to get information about how many atoms and electron pairs surround a central atom. It cannot, however, give us direct information about the geometry of the molecule, or specific information such as the bond angles of central atoms. Bond angles are angles made by the joining lines of the nuclei of the atoms in a molecule. The bond angles are determined by how many thing are around a central atom. Take for instance, CCl4, shown below.
It appears from the Lewis structure to have bond angle of 90°, however the actual bond angle are 109.5°. This is information we get by determining the geometry around the central atom (not just by looking at the Lewis structure).
ABn Geometries
For structures where a central atom is surrounded by 2 or more peripheral atoms (or electron pairs), we can give the molecule the general formula ABn, where A is the central atom, B represents the electron groups (bonded atoms or electron pairs) and n is the number of electron groups.
Most ABn geometries fall under one of the five geometries shown below.
AB2 = Linear = all electron groups fall in a line with bond angles of 180°.
AB3 = Trigonal planar = the electron groups make a triangle around the central atom and all the electron groups are in one plane with bond angles of 120°.
AB4 = Tetrahedral = the electron groups form a tetrahedron shape (4 sides) around the central atom with bond angles of 109.5°.
AB5 = Trigonal bipyramidal = the electron groups two (bi) pyramids with triangle bases surround the central atom with bond angles of 120° and 90°.
AB6 = Octahedral = the electron groups form a octahedron shape (8 sides) around the central atom with bond angles of 90°.
Valence Shell Electron Pair Repulsion (VSEPR) Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory was developed to describe the threedimensional shape of molecules. The theory centers on the repulsive nature of electrons. In VSEPR, valence electrons (bonding or nonbonding) will repel other electrons. In VSEPR, a molecule’s (or molecular ion’s) geometry is such that it minimizes electronelectron repulsions.
The two types of geometries described by VSEPR are the electronpair geometry (also called electron domain geometry) and molecular geometry. The electron domain geometry is determined by the number of electron groups (or domains) surrounding a central atom. For example, individual bonds (single, double, or triple) and each pair of nonbonding electrons (lone pair electrons) are each considered a single electron domain. The other type of geometry is the molecular geometry (also shown in the figure). The molecular geometry attempts to describe the geometry of just the atoms of the molecule. Below is a figure that summarizes all the geometries and their bond angles.
Information about VSEPR

The electron pairs and atoms around a central atom will give a molecule its shape.

Electron pairs around a central atom in a molecule will attempt to be as far away from each other as possible.

Nonbonding electron pairs around the central atom will have an effect on the shape of the entire molecule, very similar to how an atom would.

The “ideal” bond angle is found using the electron domain geometry, where the bond angle is the angle between a bonded atom, the central atom, and another bonded atom.

Note: Nonbonding electron pairs repel more strongly than bonding electron pairs (bonds), causing a difference in the “ideal” bond angle. Additionally, multiple pairs of shared electrons (bonds) repel more strongly than a single pair of electrons. Therefore, the magnitude of repulsion is as follows: nonbonding pair > triple bond > double bond > single bond.

Steps for VSEPR

Draw a Lewis structure for the ion or molecule in question.

Determine the number of electron groups/domains around the central atom. Each lone pair or nonbonding pair of electrons counts as a single group. Likewise, each bond counts as a single group, even if it is a double or triple bond. Find the corresponding electron geometry from the table.

Determine the number of lone pairs and the number of bonding pairs around the central atom, and use that to find the molecular geometry.