Lewis structures of molecules are two dimensional, giving only the image of bonding in the molecule. But, all molecules containing three or more atoms are three dimensional in nature.
The shape of a particular molecule is determined by the specific arrangement of atoms in it and the bond angles. Molecular shapes may be of many types like.
- Bent or angular
- Trigonal planar
- Trigonal bi pyramidal
The shapes of the molecules can be determined in the laboratory by modern methods such as x-ray and electron diffraction techniques. Molecular shapes are important because they are helpful in the investigation of molecular polarity, molecular symmetry or asymmetry.
Physical and chemical properties of compounds depend on these factors. VSEPR theory throws light on the three dimensional shapes of molecules.
According to this theory,
“The electron pairs (Both lone pairs and shared electron pairs) surrounding the central atom will be arranged in space as far apart as possible to minimize the electrostatic repulsion between them”.
Let us consider the simple case of two electron pairs. To place them as far apart as possible in a sphere, we have to place them at 180o from each other. That would make it a straight line. And, this arrangement of electron pairs at 180 degrees is said to be linear arrangement, because the electron pairs and the central atom are in a straight line.
Valence shell electron repulsion theory or more commonly called as VSEPR theory, is simple but powerful model for predicting molecular geometries of the molecules and bond angles of these molecules.
The structure of Water Molecule has given Below:-
Such a molecule is called a bent molecule or angular molecule. The hybridization in such molecules would be sp3.
- In the structural formula of water, the oxygen atom is bonded to two H atoms by covalent bonds and has two lone pairs.
- Thus, oxygen atom is surrounded by two bonding pairs and two unshared electron pairs. VSEPR theory says that in order to secure maximum separation between then, the four electron pairs are directed to the corners of a tetrahedron.
- If we look at the three atoms, the atoms H-OH line in the same plane and the predicted bond angle is 109.5o.
- But with the two unshared electron pairs repelling the bonding pairs, the bond angle is compressed to 105o, the experimental value. Thus, H2O molecule is flat and bent at an angle at oxygen atom.
Octahedral compounds are seen in such molecules where the central atom has two ‘d’ orbitals combining with one ‘s’ and three ‘p’ type orbitals, making up for six hybrid orbitals.
So, according to the VSEPR theory, to be as far away from one another, four of them are dispersed at an angle of 90 degrees each and the rest two are directed up and below this plane in a direction perpendicular to it.
- In the SF6 molecule, sulfur atom has‘d’ orbitals participating in the hybridization and thus has octahedral shape.
- The angle of an octahedral molecule is 90o. Four of the hybrid orbitals lie in one plane and are placed 90o from each other. The other two hybrid orbitals lie above and below the plane.
- The six fluorine atoms may approach at the six corners of the octahedron for an overlap.
- In PCl5 molecule, phosphorus is the central atom shows dsp3 hybridization. Three if these five bonding orbitals lie in a plane inclined at an angle of 120o, while the other two are directed above and below the plane in a direction perpendicular to the plane, taking the shape of a trigonal bipyramid.
- These orbitals of phosphorus atom can overlap with those of five chlorine atoms forming the PCl5 molecule which will therefore have trigonal bipyramidal shape as shown above.
- Here, some bond angles are 90o while other bonds have an angle of 120o between them. This geometry of the molecule explains high reactivity of two of the five Cl atoms in PCl5.
The Lewis structure of NH3, ammonia molecule shows that the central N atom has three bonding electron and one lose electron pair.
- The VSEPR theory says that these electron pairs are directed at the corners of a tetrahedron. Thus, the bond angle of such molecules is predicted as 109.5o.
- But the shape of the molecules is determined by the arrangement of atoms and not by the unshared electrons.
- Thus, if we see only at the atoms, we can visualize ammonia molecule as a pyramid with the N atoms located at the apex and H atoms at the three corner of the triangular base.
- According to the VSEPR theory, a lone pair exerts greater repulsion on the bonding electron pairs than the bonding pairs do on each other. As a result, the bonds of NH3 molecule are pushed slightly closer.
- This explains why the observed bond angle H-N-H is found to be 107.3o, instead of 109.5o predicted for tetrahedral geometry.
- All molecules in which the N atom is joined to three other atoms by covalent bonds, have pyramidal shape. For example, primary amines(R-NH2), secondary amines (R2NH) and tertiary amines (R3N) all have pyramidal shape.
As according to the VSEPR theory, the bonding pairs and the lone pairs should be at a maximum distance from each other, each type of hybridization has a definite bond angle, except for some cases where there is a change in the bond angle due to lone pair of electrons.