The valence shell electron pair repulsion (VSEPR) theory is a model used to predict 3-D molecular geometry based on the number of valence shell electron bond pairs among the atoms in a molecule or ion. This model assumes that electron pairs will arrange themselves to minimize repulsion effects from one another. In other words, the electron pairs are as far apart as possible.
The VSEPR model is useful for predicting and visualizing molecular structures. The structures are: linear, trigonal planar, angled, tetrahedral, trigonal pyramidal, trigonal bipyramidal, disphenoidal (seesaw), t-shaped, octahedral, square pyramidal, square planar, and pentagonal bipyramidal.
The VSEPR structures take the names of 3-D geometric shapes, as in the example trigonal bipyramidal. Under the VSEPR model, a trigonal bipyramidal molecule such as phosphorus pentachloride or PCl5, with a central phosphorus atom and five valence shell electron pairs, looks like two (bi) connected triangular-base pyramids, where each atom is the vertex or corner of a triangular face.
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To use a VSEPR table, first determine the coordination number or number of electron pairs.
Finally, look up your molecule on the chart by coordination number and number of atoms.
Alternatively, you can count the lone electron pairs, which are also indicated on the chart.
Once you know PCl5 has five electron pairs, you can identify it on a VSEPR chart as a molecule with a trigonal bipyramidal molecular geometry. Its bond angles are 90 ° and 120 °, where the equatorial-equatorial bonds are 120 ° apart from one another, and all other angles are 90 °.
Some other examples shown on the VSEPR chart are sulfur hexafluoride, SF6, whose six electron pairs give it octahedral geometry with 90 ° angles, and CO2, which has two electron pairs and linear geometry.
VSEPR is an acronym that stands for valence shell electron pair repulsion. The model was proposed by Nevil Sidgwick and Herbert Powell in 1940. Ronald Gillespie and Ronald Nyholm then developed the model into their theory published in 1957; they are considered the developers of the VSEPR theory. The approach was commonly referred to as VSEPR from 1963 to the present.
Gillespie summarizes the VSEPR theory rules as:
Nonbinding domains are larger than single bond domains; they are more spread out and occupy more space in the valence shell than single bond domains. This is understandable because lone pairs are under the influence of only one positive core rather than two.
The size of a single bond domain in the valence shell of a central atom decreases with increasing electronegativity of the ligand.
Although it is often convenient to think of double and triple bonds as composed of a σ or two π bonds or two or three bent single bonds, respectively, it is simpler in the electron pair domain model to consider a double bond as a two electron pair domain and a triple bond as a three electron pair domain in which the individual electron pairs are not distinguished. These bond domains increase in size from a single to a double to a triple bond.1
VSEPR is often explained to beginners as eight simpler postulates:
Molecular geometry is a method to determine the shape of a molecule based on the repulsion occurring between bond electron pairs in the outermost (or valence) electron shell. It’s useful to study molecular geometry to get information beyond that provided in a Lewis structure. Many physical and chemical properties are affected by the shape of molecules.
VSEPR is a molecular geometry model that helps predict the general shape of a molecule but doesn’t provide information about the length or type of bonds. VSEPR theory is not effective in molecules where the central atom is a transition metal and thus has a high atomic mass that offsets or weakens the pull of bonded valence electrons.
The VSEPR model is one way to determine molecular geometry. A more advanced way of determining the shape of a compound is electron geometry. Both approaches depend on information about electrons, but the electron geometry model accounts for all electrons. The two models can predict different shapes for the same molecule.
You can also differentiate the two by thinking of electron geometry as a way of looking at the electrons that surround an atom and molecular geometry as a way of looking at the arrangement of atoms around a central atom.
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