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Valence-Shell Electron-Pair Repulsion (VSEPR) Model

Molecular Geometry

The water molecule consists of an oxygen atom covalently bound to two hydrogen atoms. What is the shape of the water molecule? Do the atoms lie on a straight line (linear geometry)? Is the molecule V-shaped?

The shape of a molecule has profound implications for is properties and reactivity. If the water molecule has the shape of a V (with the oxygen at the lower vertex and hydrogens at the upper left and right), the molecule will be polar. The lower part of the molecule will have a partial negative charge (because the oxygen strongly attracts shared electrons to itself), while the top part of the molecule will have a partial positive charge (because hydrogen atoms have a weaker pull than oxygen on shared electrons). If the water molecule is linear, the molecule will be non-polar. That is, there will not be a negative end and positive end of the molecule.

Many properties of a compound, such as its ability to serve as a solvent, the melting and boiling points, and heats of fusion and vaporization, depend strongly upon its polarity.

Electron Pairs

An essential tool for chemists is a simple, reliable strategy for determining the shapes of small molecules and portions of larger molecules. This tool is called the Valence-Shell Electron-Pair Repulsion Model (VSEPR model). The key concept in the VESPR model is that electrons, each having a negative charge, repel each other. The molecule will adopt whatever geometry minimizes the energy of these repulsions. An alternate way of thinking is to say that the electrons want to get as far away from each other as possible.

1. Write the best Lewis structure for the molecule.

The starting point for the VSEPR Model is to determine the connectivity of the atoms and write a good Lewis structure for the molecule. Most stable molecules contain an even number of electrons and the electrons occupy orbitals as pairs (one spin-up and one spin-down). Thus the Lewis structure depicts electrons as pairs, either in bonds or as lone pairs (sometimes called non-bonding pairs). Also recall that a Lewis structure shows only the valence electrons (no core electrons). At this point the terms in the name VSEPR should now make sense.

1. Write the best Lewis structure for the molecule.
2. Determine number of electron groups (NEG) around the central atom.

With a good Lewis structure in hand, identify the central atom (for water this is oxygen) and determine the number of Electron Groups around the central atom. The number of Electron Groups equals the total number of lone pairs (NLP) and sigma bonds (NB) around the central atom. Note that electron pairs in pi bonds are not counted. The electron pair in a π bond is displaced from the region immediately between the two bonded atoms and thus is not a significant source of electron-pair repulsion in the system. The primary sources of electron-pair repulsion arise from lone pairs and sigma bonds. For the purpose of determining the number of Electron Groups, count an entire bond (single, double, or triple) as a single Electron Group.

NEG = NLP + NB

Electron Group Geometry

1. Write the best Lewis structure for the molecule.
2. Determine number of Electron Groups (NEG) around the central atom.
3. Use NEG to determine the geometry of the Electron Groups.

Once the number of Electron Groups (NEG) is known, one can predict the geometry of the Electron Groups. The electron pairs (lone pairs and sigma bonding pairs) will assume whatever geometry minimizes their mutual repulsion (Coulomb's law). With a knowledge of trigonometry and calculus, one can readily solve this minimization problem. The results are shown in the following table. For all but the largest atoms (when bound to small groups), there is insufficient space to accommodate more than six Electron Groups around the central atom. Thus the table accounts for all commonly encountered geometries.

NEG Electron Group Geometry
2linear
3trigonal planar
4tetrahedral
5trigonal bipyramidal
6octahedral

Molecular Geometry

1. Write the best Lewis structure for the molecule.
2. Determine number of Electron Groups (NEG) around the central atom.
3. Use NEG to determine the geometry of the Electron Groups.
4. Identify which Electron Groups are associated with bonds and determine the molecular geometry.

Once the Electron Group geometry is determined, one can finally predict the molecular geometry. The molecular geometry describes the relative positions of the atoms in the molecules. Thus one must determine which Electron Groups are associated with an atom (an atom other than the central atom) and which are not. Electron Groups attributable to lone (non-bonding) pairs of electrons (NLP) have no atom. Electron Groups attributable to bonds (NB) do have an atom. Recall that

NEG = NLP + NB

The various possible combinations NEG and NB are tabulated below. In each case, an example is provided. Click on the chemical formula to display the molecule. The geometry of the molecule should be fairly similar to the description in the table.

Carefully examine the molecular geometry where NEG = NB (highlighted in blue) to visualize the Electron Group geometry. In this case all Electron Groups are associated with atoms and the Electron Group geometry is identical to the molecular geometry.

Then look at examples where NEG > NB. In this case some of the Electron Groups are lone pairs, and the molecular geometry has lower symmetry than the Electron Group geometry. (In most cases where NEG > NB, it does not matter which site is occupied by an atom. The exceptions are for NEG = 5 or 6. See the next page on this topic for a discussion of which sites are occupied by atoms and which by lone pairs.)

As a technique for determining molecular geometry, the VSEPR Model is simplistic and lacks rigorous theoretical foundation. Nonetheless, the VSEPR Model is remarkably good at predicting actual molecular geometries. The geometries shown in the table represent idealized geometries, and real molecules often show deviations from the ideal bond angles listed above. Fortunately, it is easy to anticipate why such deviations occur. The next page in this tutorial explains the origin of these deviations from the ideal geometry.

Finally, note that the geometry of the Electron Groups is closely tied to the hybridization of the central atom, because the electrons reside in orbitals and thus the central atom must have valence-shell orbitals with orientations that coincide with the orientations of the Electron Groups predicted by the VSEPR Model. (This is a simple version of valence bond theory. More advanced versions of the theory do not have this requirement.) The table also lists the hybridization scheme associated with each geometry.

Click on the molecular formula to display the molecular structure.

NEG NB NLP Molecular Geometry Ideal Bond Angle(s) Hybridization Example
220linear 180° sp
CO2
321V-shaped 120° sp2
O3
330trigonal planar 120° sp2
BF3
422V-shaped 109.5° sp3
H2O
431trigonal pyramidal 109.5° sp3
NH3
440tetrahedral 109.5° sp3
CH4
523linear 180° dsp3
I3-
532T-shaped 90° dsp3
ClF3
541see-saw 90°, 120° dsp3
SF4
550trigonal bipyramidal 90°, 120° dsp3
PCl5
642square planar 90° d2sp3
ICl4-
651square pyramidal 90° d2sp3
BrF5
660octahedral 90° d2sp3
PF6-
                       

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Deviations from the Ideal Geometry             VSEPR Model Exercises



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© Copyright 2012-2014 David N. Blauch