The+VSEPR+Model

The **valence-shell electron-pair repulsion**, also known as **VSEPR**, model is one that accounts for the geometric arrangements of shared and unshared electron pairs around a central atom in terms of the repulsions between electron pairs.

//Bonding pairs of electrons// represent the area, or **domain**, that the electrons will most likely be found. //Nonbonding pairs of electrons//, or **lone pairs**, represent a domain that is located principally on one central atom.

__Example:__ The Lewis Structure for CO 2 can be drawn as: in which the yellow ovals encircle the lone pairs, and the blue boxes contain bonded pairs.

The geometric shapes of molecules are determined by bond angles, which in turn are determined by the VSEPR model.

Two ideas to keep in mind when considering the VSEPR model are:


 * a)** Electron domains repel and remain as distant as possible to minimize this repulsion.
 * b)** Nonbonded electron pairs take up more space than bonded electrons.

Single bonds consist of 2 electrons that are shared between the atoms, as a covalent bond. //Each bond, double bond, or multiple bond is considered as one domain around the central atom.//

__Example:__ The Lewis Structure for O 3 can be drawn as:

 The double bond between the O atoms on the right is considered as one domain; therefore, O 3 has a total of 3 domains: the single bond, the double bond, and the lone pair.

__**Electron-Domain Geometry**__ Electron- domains are arranged to minimize repulsions. The number of domains surrounding the central atom account for the **electron- domain geometry**, which is defined as the arrangement of electron- domains about the central atom of a molecule. Central atoms with two electron- domains are considered to be linear; those with three are trigonal planar; four are tetrahedral; five are trigonal bipyramidal; and six are octahedral. Each arrangement of electron- domains depends on the number of bonded and nonbonded electrons, and thus, deciphers the bond angles. The following table summarizes electron- domain geometries and their predicted bond angles. The **molecular geometry** is the arrangement of only the atoms in a molecule or ion (nonbonding pairs are not part of this description).
 * __Molecular Geometry__**

The following steps can be used to predict the shapes of molecules or ions:


 * 1)** Draw the Lewis structure of the molecule/ion. Count the total number of electron domains around the central atom. (Remember, //each lone pair, single bond, or multiple bond is considered as one domain around the central atom//.)


 * 2)** Determine the electron- domain geometry by placing the electron domains about the central atom in a way that minimizes the repulsions among the electrons.


 * 3)** Using the arrangement of the bonded atoms, the molecular geometry can be determined.

The following tables summarize all possible molecular geometries, taking into consideration the number of bonded and nonbonded domains.



The tables above summarize the ideal geometries of molecules; however, as always, there are exceptions and slight variations to the bond angles. These are :


 * 1)** //Nonbonding electrons, or lone pairs, cause more repulsion between atoms; therefore, the angles between the central atom and the adjacent atoms are compressed.//

The following image illustrates how the nonbonding pair of electrons has a low nuclear pull in comparison to the bonded electron pair, which is attracted to the nuclei. Consider the following molecules: 

As the number of lone pairs increases, the bond angle degrees decrease.


 * 2)** //Electron domains of multiple bonds exert greater repulsion on adjacent angles than do single bonds.//

The following picture depicts how the double bond between the C and O atoms increases repulsion between the central C atom and its adjacent Cl atoms, thus decreasing the bond angle between the Cl- C- Cl atoms from 120° to 111.4°.



The VSEPR model can be used not only for molecules with one central atom, but also for compounds consisting of more than one central atom. Consider for example acetic acid:



Starting from the left, the central C atom has a total of four domains: the three C-H single bonds and the C-C single bond. Its electron-domain geometry, according to Table 9.2, is tetrahedral, and thus, its predicted bond angles are 109.5°. The next central C atom has a total of three domains: the C-C single bond, the C=O double bond (remember, multiple bonds are considered as one domain), and the C-O single bond. The electron-domain geometry is trigonal planar, and its predicted bond angles are 120°. The central O atom has four domains: the C-O single bond, the O-H single bond, and when drawing out the Lewis structure, O will have two lone pairs, one above and one below the O atom. Therefore, its electron-domain geometry is tetrahedral, and its predicted bond angles are 109.5°. Since the molecule has lone pairs, it can be considered as tetrahedral bent, and thus, have bond angles that deviate from the expected 109.5°.

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