Any chemical compound that is formed with either ionic or covalent bond follows certain pattern in molecular arrangement. This arrangement gives the molecule a maximum stability and this stability is as much important for the compound as the chemical stability. The spatial distribution takes in to account the charge factor, stereo factor and symmetry factor in to consideration. This is called the configuration.
In this arrangement, not only the bonded electrons take place but there is an involvement of the non bonded electrons too. Non bonding electrons distort the bonds so that their movement is not hindered.
- Van't Hoff and Lebel in 1874 first proposed independently that the carbon atom in methane (CH4) has its four bonds directed towards the corners of a regular tetrahedron. This assumption is made to explain the existence of the isomers.
- Isomers are compounds that will have same chemical formula but different physical properties and chemical properties. For example Maleic acid (m.p=1310C) and Fumeric acid(m.p=3070C). These are called optical isomers.
- Molecules are multi dimensional. By this we mean that in a molecule there are many bond angles and bond distances. A simple chemical formula at the most can give us the information about the connections between atoms in a molecule.
- But the way that a molecule fills the space is determined by the ways all the various distances and angles fit to make a real molecule. To understand this aspect we often make use of the models.
- Experimental determination of the shapes of the molecule is close to the actual position with a variation of about 10%. This is because of the assumption that the nucleus is stationary along with the inner electrons and only the valence electrons that are involved in bond formation or not, are the ones that adjust to the spatial requirement.
- The shapes of the smaller molecules give an idea of the shapes of the larger molecules which are most of the times a simple extension or multiples of the simple molecules.
- Looking at the molecules in terms of the bond between the atoms one should concentrate on the central atom bonding with all surrounding atoms of the molecule.
The following varieties of molecules have their own shapes and orientations. Let us see the details.
AX2 Type : AX2 type molecules have two geometrical arrangements. Linear (CO2, BeH2) and bent (H2O, OF2, SO2 and CF2 etc). The presence of non bonding pair of electrons gives the shape a bent shape.
AX3 Type : This type of molecules shows three types of geometry. Trigonal planar, trigonal pyramidal, and an unusual T shaped molecule. The T shaped molecule (BrF3) is unusual shape occurring only when the electron density is more around a larger central atom with electron density.
AX4 Type : This class of compounds are in perfect tetrahedral geometry, square planar geometry and the unusual distorted tetrahedral geometry (SF4). The distorted tetrahedral geometry is due to the density of electrons on fluorine atoms.In this one of the usual tetrahedral angles has enlarged to nearly 180o which shrinks the other to less than 109.5o.
AX5 Type : Trigonal bipyramidal and square pyramidal types of two geometry is seen in this AX4 type compounds. If the central atom is smaller the surrounding atoms get distorted due to electron density and form trigonal bi pyramid (PF5) while the if the central atom is larger or having a number of lone pairs of electrons the distribution is more symmetrical in square planar pyramidal structure (BrF5).
AX6 Type : Octahedral is the only one important shape with this type of molecules and the molecule is stable in the geometry.
There are more molecules where there are more than six atoms bonded to the central atom but their shapes are not fixed and some of them are flexible in nature with regards to the geometry.
||α =102°, β= 187°
||α =90°, β= 120°
||Trigonal bi pyramid
||α =87°, β= 90°