VSEPR stands for Valence Shell Electron Pair Repulsion. That's a real mouthful for what is really a rather simple idea.
The whole concept revolves around the idea that the electrons in a molecule repel each other and will try and get as far away from each other as possible. VSEPR explains a lot about molecular geometry and structure, BUT NOT EVERYTHING!!
The electrons (both in pairs and singles as you will see) are "attached" to a central atom in the molecule and can "pivot" freely on the atom's surface to move away from the other electrons.
Electrons will come in several flavors:
a) bonding pairs - this set of two electrons is involved in a bond, so we will write the two dots BETWEEN two atoms. This applies to single, double, and triple bonds.
b) nonbonding pairs - this should be rather obvious.
c) single electrons - in almost every case, this single electron will be nonbonding.
Almost 100% of the examples will involve pairs, but there are a significant number of examples that involve a lone electron.
VSEPR uses a set of letters to represent general formulas of compounds. These are:
a) A - this is the central atom of the molecule (or portion of a large molecule being focused on).
b) X - this letter represents the ligands or atoms attached to the central atom. No distinction is made between atoms of different elements. For example, AX4 can refer to CH4 or to CCl4.
c) E - this stands for nonbonding electron pairs.
d) e - this stands for lone nonbonding electrons.
Each area where electrons exist is called an "electron domain" or simply "domain." It does not matter how many electrons are present, from one to six, it is still just one domain. Now a domain with six electrons in it (a triple bond) is bigger (and more repulsive) than a lone-electron domain. However, it is still just one domain.
This is an IMPORTANT point to remember in VSEPR. The more electrons in a domain, the more repulsive it is and it will push other domains farther away than if all domins were equal in strength. Keep in mind that the domains are all attached to the central atom and will pivot so as to maximize the distance between domains.
Another important point to mention in this introduction is that an element's electronegativity will play an important role is determining its role in the molecule.
For example, the least electronegative element will be the central atom in the molecule. The more electronegative the element, the more attractive it is to its bonding electrons This will play a very important role, especially in five domains.
The most important domain numbers at the introductory level are 3, 4, 5, and 6. Domains of 1 and 2 exist, but are simple to figure out. We'll do them here in the ChemTeam. Domains up to 9 exists, but become progressively more complex. If you decide you MUST study those domains, seek out this book:
The VSEPR Model of Molecular Geometry (1991) by Ronald J. Gillespie and Istvàn Hargittai. Published by Allyn & Bacon. ISBN 0-205-12369-4.
Gillespie coined the term VSEPR and has been active in this field since it was established in the early 1940's. Except where noted, all bond angles and bond lengths have been taken from this book.
There are two sites on the web which have excellent VSPER materials. They are:
VSEPR Site #1
VSEPR Site #2
Visit the first site and use it in conjunction with the ChemTeam. You will need to download a Netscape plug-in called Chime. The second site is more oriented to practice testing.
Copyright © Dr. Donald L. Robertson (Modified: 09/13/2009)