The valence shell electron pair repulsion (VSEPR) model is based on the observation that the geometrical arrangement of bonds around an atom is influenced by non bonding electrons present. It is assumed that electron pairs - whether bonding or non bonding repel each other and adopt a geometrical arrangement that maximizes the distances between them.
The valence shell electron pair repulsion is based on the idea that both bonding and non bonding electron pairs in the valence shell of an atom "repel" each other. When non bonding electron pairs are present in the valence structure however less regular arrangements of bonds are often found. According to this theory, the direction of the bonds around an atom in a molecule depends upon the total number of electron pairs (bonding as well as non-bonding) in the valence shell of an atom. Since the electron pairs repel each other therefore that geometrical arrangement will be favoured in which repulsion is minimum.
VSEPR Model DefinitionBack to Top
The best arrangements of a given number of electron pair is the one that minimizes the repulsion among them. Hence according to the theory "the orbitals occupied by electrons in the valence shell of the central atom arrange themselves in space in a way that they lie far away from one another as possible to impart minimum repulsion in them".
VSEPR Model ChartBack to Top
The valence shell is the outermost electron occupied shell of an atom and usually holds the electrons that are involved in bonding. Pairs of electrons which are shared hold atoms together in molecules and are known as "bonding pairs". The VSEPR model is used to predict how the electron clouds of atoms within compounds repel each other and results in an overall molecular geometry.
In attempting to predict molecular geometry
- draw the Lewis structure
- count the number of electron pairs (both bonding and lone pairs) and
- use the following chart to "key out" the geometry
VSEPR ModelsBack to Top
The VSEPR model is a reliable method for predicting the shapes of covalent molecules and polyatomic ions. This model is based on the idea that bond and lone pairs in the valence shell of an element repel each other and seek to be as far apart as possible. The position assumed by the valence electrons of an atom thus define the angles between bonds to surroundings atoms. VSEPR is remarkably successful in predicting structures of molecules and ions of main group elements. However it is less effective to predict structures of compounds containing transition metals.
Molecules of any size can be treated by applying the VSEPR model to each appropriate atom in the molecule. Thus this model is used to predict the structures of molecules with hundreds of atoms.
VSEPR TheoryBack to Top
The following rules are helpful in using the VSEPR model to predict molecular structure.
- Determine the Lewis structure for the molecule.
- For molecules with resonance structures use any of the structures to predict the molecular structure.
- Sum the electron pairs around the central atom.
- When counting pairs count each multiple bond as a single effective pair.
- Determine the arrangement of the pairs that minimizes electron pair repulsions.
- Lone pairs require more space has bonding pairs. Choose an arrangement that gives the lone pairs as much room as possible, although it appears that an angle of at least 120 degrees between lone pairs provides enough space. Recognize that lone pairs at angles less than 120 degrees may produce distortions from the idealized structure.
The VSEPR model explains the observed geometry of molecules by an electrostatic picture based primarily on the Lewis structure. The model assumes that each electron pair in the valence shell of an atom in a molecule occupies a well defined region of space. The first and most important rule of VSEPR is that the bond angles about an atom are those that minimize the total repulsions between the electron pairs in the valence shell of the atom.
VSEPR Theory ChartBack to Top
Valence shell electron pair repulsion permits the geometries arrangement of atoms, or groups of atoms, about some central atom to be determined by considering the repulsion between the electron pairs present in the valence shell of the central atom. Based on VSEPR the general shape of any molecule can be predicted from the number of bonding and non bonding electron pairs in the valence shell of the central atom, recalling that non bonded pairs of electrons are more repellent than bonded pairs.
The approximate geometry of the atoms around a central atom can be predicted by the valence shell electron pair repulsion theory. This theory tells us that electron pairs either un shared pairs or electrons localized in a bond repel each other to the maximum extent. In other words the negatively charged electron groups get as far away from each other as possible because of their electrostatic repulsion. The geometry assumed by the un shared electron pairs and the electrons in the bond is known as electronic geometry.
VSEPR Theory ExamplesBack to Top
Shape of BeF2 molecule: In BeF2, the central atom Be has 2 valence electrons and forms 2 covalent bonds with the 2 fluorine atoms. According to VSEPR theory, these bond pairs arrange themselves at an angle of 180o and the molecule will be linear.
Shape of BF3: Boron the central metal atom has 3 electrons in the valence shell and forms 3 covalent bonds with 3 fluorine atoms which are directed towards the corners of a regular equilateral triangle leading to a bond angle of 120o.
Shape of CH4: Carbon atom has 4 valence electrons which form 4 covalent bonds with the 4 hydrogen atoms leading to a regular tertrahedral shape, with a bond angle of 109o28'.
VSEPR Theory ShapesBack to Top
The VSEPR method is applied to predict the shape of a molecule or polyatomic ion.
- Derive the Lewis structure of the species using the rules given. If the species displays resonance, choose one resonance form.
- Identify the central atom. The central atom is the one to which all other atoms are joined.
- Determine the number of electron domains about the central atom by totaling the number of non bonding pairs and bonds about the central atom. Single, double and triple bonds all count as one domain. The number of domains predicts a specific shape of the species.
Frequently the three dimensional shape of the molecule, the arrangement of just the atoms is not described with the same label used for the arrangement of the electron domains about the central atom.
VSEPR Theory Bond AnglesBack to Top
The bond angle is the angle of arc between any two bonds that have one atom in common. The bond angle better indicates the three dimensional shape of a species. Three of the most important bond angles are shown in the following three models.
These geometric arrangements has associated with a defined bond angle. It describes how different numbers of valence electrons around a central atom adopt characteristic geometries. To use VSEPR theory we count the number of pairs of bonding electrons plus the number of non bonding pairs around the central atom.