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Valence-Shell Electron-Pair Repulsion Models

Skills to be tested

  1. Identify the central atom in a molecule containing more than two atoms as a start.
  2. Identify the number of valence electrons of any element. This concept is important, because you need to know the number of valence electrons in order to write a Lewis dot structure for the molecule in question.
  3. Count the number of VSEPR pairs or steric number (SN) for the central atom in a molecule. You need this number in order to describe or predict the shape of the molecule in question.
  4. Determine the number of lone electron pairs that are not shared with other atoms. Often, a Lewis dot structure is useful to help you count this number.
  5. Predict the shape of molecules or inos as the key concept of VSEPR theory. From the shape and by applying the idea that lone electron pairs takes up more space, you can predict the bond angles withing 5% of the observed values.
  6. Predict the values of bond angles and describe the hybrid orbitals used by the central atoms in the molecules or ions.

Valence-Shell Electron-Pair Repulsion (VSEPR) Models

The 3-dimensional structure of BF3 is different from PF3, and this is difficult to comphrend by considering their formulas alone. However, the Lewis dot structure for them are different, and the electron pair in :PF3 is the reason for its structure being different from BF3 (no lone pair).

Three-dimensional arrangements of atoms or bonds in molecules are important properties as are bondlengths, bond angles and bond energies. The Lewis dot symbols led us to see the non-bonding electron pairs, whose role in determining the shape of a molecule was examined by N.V. Sidgwick and H.E. Powell in 1940, and later by R.S. Nyholm and R.J. Gillespie. They have developed an extensive rationale called valence-shell electron-pair repulsion (VSEPR) model of molecular geometry.

Molecular shapes and steric numbers (SN)
Example SN Descriptor
BeCl2, CO2 2 Linear
BF3, SO3
3 Trigonal planar
4 Tetrahedral
5 Trigonal bypyramidal
6 octahedral
square planar
E represents a lone electron pair.
SN is also called the number of VSEPR pairs
or number of electron pairs.
The Valence-Shell Electron-Pair Repulsion (VSEPR) models consider the unshared pairs (or lone electron pairs) and the bonding electrons. This considerations of lone and bonding electron pairs give an excellent explanation about the molecular shapes. The VSEPR model counts both bonding and nonbonding (lone) electron pairs, and call the total number of pairs the steric number (SN). If the element A has m atoms bonded to it and n nonbonding pairs, then

SN = m + n SN is useful for predicting shapes of molecules. If X is any atom bonded to A (in single, double, or tripple bond), a molecule may be represented by AXmEn where E denote a lone electron pair. This formula enable us to predict its geometry. The common SN, descriptor, and examples are given in the table on the right.

Note that the SN is also called the number of VSEPR pairs or number of electron pairs. The VSEPR model has another general rule:

lone pairs of electrons take up more space than bonded pairs making the bond angle, say H-O-H for water less than the tetrahedral angle of 109.5 °. Actually, the H-O-H angle in water is 105 °.
The geometry of the molecules with their SNs equal to 2 to 6 are given in the Table. The frist line for each is the sape including the lone electron pair(s). If the lone electron pairs are ignored, the geometry of the molecule is given by another descriptor.

To get an idea about the shapes of molecules and ions, three dimensional models are the best to use. However, good computer graphics sometimes also illustrate very well. The link VSEPR Illustration: View and manipulate molecular models gives excellent graphics, and you may enjoy seeing some of the graphics of the molecules.

Another link showing beautiful photographs of models of Molecular Geometry is also interesting. One of its photograph has been used in the previous page Hybrid orbitals.

VSEPR in French

Molecular database without transition elements is a very nice site in French, and it is very interesting to read for those who does not know French.

Confidence Building Questions