Shape of Molecules

We can determine the location of electrons and nuclei of an atom with molecular geometry and with the help of location of electrons and nuclei we can determine the shape of molecules. for example – H2O (water)

We know about the location of electrons and nuclei in molecules of water then we can easily determine that H2O molecules bent or V shape.In an atom nuclei and electrons occupies that position from where they can decrease the force to repulsion between electrons and increases the force of attraction between electrons and nuclei. Shape of molecules also helps in stabilising an atom and molecular geometry plays a very important role in determining the shape of any molecule.Shape of molecules reflects the shape of atom where they tend in equilibrium. The state which molecule has less or we can say lowest possible energy present in the system. 

Generally VSEPR theory is used to determine the shape of molecules and according to VSEPR theory mainly the orientation of the central atoms valence shell electrons is related in defining the shape of the molecules.The electrons present in valence shell are negatively charged therefore, electrons are constantly apply force on repulsion on each other to occupy maximum distance from each others Repulsion is the property which gives the molecules three -dimensional shape.

With the help of molecular geometry we can determine the shape of molecules and there are five different types of shape of molecules, for which we have to know about steric numbers.

Steric Number 

For the central atom of molecular dogma it is very convenient to define steric numbers.

Steric numbers provide us an easy path to  determine the shape of molecules.steric numbers are counted as number of bonded atoms +number of lone pairs.

1. Linear geometry (SN=2)

In linear geometry steric number is two and the shape of molecules are linear in shape and they possess an angle of 1800. They have sp-hybridization when carbon atoms are present.They have two bonded atoms but no lone pair is present in linear geometry.

The shape of molecules which are defined by linear geometry are: acetylene, beryllium fluoride, carbon dioxide etc.

2. Trigonal Planar geometry (SN=3)

In this type of molecular geometry the electrons groups are oriented towards the corner of an equilateral triangle. In this all the electrons groups present in bonding groups and angle formed between two bonds are 1200 and this is the ideal bond angle.In total trigonal Planar geometry have three bonds. The shape of molecules which are defined by trigonal planar geometry are: Boron trichloride(BCl3),formaldehyde (CH2O),Carbonate anion(CO32-).

3. Tetrahedral- based Structures (SN =4)

Tetrahedral-based structures have four steric number and their structures are based on tetrahedra. They have four atom bonding and the position of the bonded atom lies at the corner of the tetrahedron. The angle formed by tetrahedron based molecules are 109.50.The shape of molecules which are defined by tetrahedral are: Methane(CH4), Ammonia which have three bonding and one lone pair electrons and have bond angles 107.30 and it have trigonal pyramidal structure. Water has two bonding pairs , two lone pairs and has an angle of 104.50 and it has a bent or V shaped structure.

4.Trigonal bipyramidal-based (SN = 5) structures

Trigonal bipyramidal structure have five steric number.It have five electron domains bonding and position of the atoms bounded lies at the corner of trigonal bipyramid.In this there are two types of bond angles on at equatorial which have 900 bond angle and other one is axial which have 1200 bond angle.

In trigonal bipyramidal structure when one lone pair occupies equatorial position and there are four bonded atoms, then the shape of molecules is see-saw. The SF4  has see-saw-like molecular geometry. 

The shape of molecules which are trigonal bipyramidal-based are  :phosphorus pentachloride (PCl5) etc.

5.Octahedral geometry (SN = 6) and octahedral-based structures

Octahedral geometry has six steric numbers. The shape of octahedral-based structures are like eight connected triangles and there are 90° bond angles present between the bonds of octahedral geometry.

The shape of molecules which are defined by octahedral-based structures are: sulphur hexafluoride (SF6), bromine pentafluoride (BrF5), and xenon tetrafluoride (Xe4).

Conclusion

There are many attempts which are done by the researchers, scientists to determine the shape of molecules and molecular geometry like  earlier they have given Lewis structure it is represented by the chemical symbols for the atom but Lewis structure is unable to describe the shape of all the compounds.After this VSEPR theory comes which was given by sedgwick and Powell and this theory is very helpful in determining the shape of molecules.

In this theory multiple bonds are treated as a single electron pair. As VSEPR theory determines the shape of molecules on the basis of bonded electrons and lone pairs present in the molecules, their many physical properties which strongly affect the shape of molecules. At last we conclude that the molecules geometry that defines the shape of molecules is one of the basis to study the molecules which contribute a very large part in chemistry.