Molecule is formed by several electronic combinations of two or more atoms to the smallest identifiable unit of pure substances which can retain the chemical properties and composition of that substance by which the molecules are made in chemistry. When the chemical properties of the molecules cannot be described by a single structure but two or more structures, its true structure in chemical science is called the resonance hybrid of pure molecules. Therefore, the resonance provides the description of the electronic structure of solid, liquid, and gas molecules by the scheme of the pairing of the electron particles.
We easy to learning the formation of ionic crystalline solid molecules by forming an ionic bonding that held between two atoms of periodic table elements together by electrostatic forces. But not clear as to what forces holding two atoms together in covalent bonding. After the development of quantum mechanics, two alternative theories explain the nature of covalent bonding. Therefore, the theories that explain the bonding in polar or non-polar molecules are Valence bond theory and molecular orbital theory.
Structure of Carbon dioxide Molecule
A carbon dioxide molecule formed by one carbon and two oxygen atoms. Hence the valence shell electronic configuration of element carbon in excited state 2s1 2px1 2py1 2pz1. Oxygen atoms (normal state) 2s2 2px1 2py1 2pz1. Liner shape of O=C=O suggests that carbon atom is the central atom with sp-hydridized. Therefore, after sp-hybridization, the electronic configuration of the carbon atom becomes, (sp)1 (sp)1 2Px1 2py1. Two sp-hybrid orbitals of carbon atom overlap with two oxygen atoms to form two sigma-bonds. Another two half-filled 2p-orbitals carbon atom binding by pi bonds with two oxygen atom. Therefore, CO2 molecule linear in shape consists of two sigma and two pi bonds with three resonance hybrids. The experimental bond length between carbon-oxygen = 1.15Å.
The dipole of the resonating structure are equal and act in opposite directions. Therefore, the resultant dipole moment of the actual structure becomes zero. Hence the resonating structure of CO2 molecule equivalent with resonance bond energy 33 kcal/mole.
Structure of Boron trichloride
Born atom is the central atom in BCl3 molecule in chemistry. The valence shell electronic configuration of the boron atom in the excited state, 2s1 2px1 2pz1. On hybridization, these atomic orbitals form three sp2 hybrid orbitals. These hybridized orbitals overlap with three singly filled 3pz orbitals of the chlorine atom to form three B-Cl sigma bonds. Therefore, BCl3 molecule is a trigonal planner shape. In this molecule central atom boron with an incomplete octet. Hence BCl3 has an affinity to accept electron pair and acts as a lewis acid by forming coordinate covalent bonding with lewis bases.
Structure of Methane Molecule
Natural gas Methane is the simplest hydrocarbon of alkane or paraffin with molecular formula CH4. In methane molecule, carbon atom exited with 2px1 2py1 2pz1 electronic configuration. On mixing these four atomic orbitals give four equivalent sp3-hybrid orbitals. These hybrid orbitals directed towards the corners of the regular tetrahedron. Therefore, each sp3-hybrid orbitals overlap with the 1s-atomic orbitals of four hydrogen atom by the formation of four sigma bonds.
Ammonia, Water, Hydrogen Fluoride Molecules
In chemistry, NH3 molecule nitrogen atom is the central atom with valence shell electronic configuration, 2s2 2px1 2py1 2pz1. These four atomic orbitals on hybridization to give four equivalent sp3-hybrid energy levels. Therefore, three of these four orbitals overlap with 1s-orbitals by uses hydrogen to form three sigma-bond. Here the remaining hybrid orbitals contain a lone pair of electrons acts as a Lewis base. Hence ammonia molecule has distorted tetrahedral shape with bond angle 107.5º.
On the basis of these agreements, water and hydrogen fluoride molecules are also formed by sp3-hybridization. Oxygen and fluorine atoms are the central atoms in these molecules with bonding hydrogen. Hence the electronic configuration of oxygen and fluorine, 2s2, 2px2 2py1 2pz1, and 2s2 2px2 2py2 2pz1 respectively. These two molecules contain two or three lone pairs respectively. Due to the presence of lone pairs the shape of the molecules is not regular tetrahedral.