Molecule, formed by two or more atoms contains the smallest identifiable unit of pure substances in chemistry and biology or science which retain the composition and chemical properties of formation substances. When the chemical properties of the molecules cannot be described by a single structural diagram, it forms different types of molecular structures, the true structure in chemical science meaning the resonance hybrid of the pure molecule. Therefore, resonance define the description of different types of electronic structures in chemistry for solid, liquid, and gas molecules by the scheme of the pairing of the electron particles.
We easy to learning the facts of ionic crystalline solid molecules by the formation of ionic bonding that combine between two atoms of periodic table element together, form by the electrostatic force of attraction. But the fact is not clear, what is the arrangement of atoms within the covalent bonding molecule.
After the development of quantum mechanics, two alternative facts explain the nature and structure of different types of covalent bonding in the molecule. Valence bond theory and molecular orbital model are examples that explain the facts of molecular structure and bonding in the polar or non-polar molecule.
Structure of Carbon dioxide Molecule
A carbon dioxide molecule is an example, formed by the components like one carbon and two oxygen atom having linear structural formula. The valence shell electronic configuration of chemical element carbon in excited state 2s1 2px1 2py1 2pz1. Oxygen atoms (normal state) 2s2 2px1 2py1 2pz1. The liner shape of O=C=O suggests that the 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, carbon dioxide molecule linear in shape consists of two sigma and two pi bonds with three resonance hybrids. The experimental bond length between carbon-oxygen in carbon dioxide molecule = 1.15Å. The dipole moment of the resonating structure are equal and act in opposite directions. Therefore, the resultant dipole moment of the actual structure becomes zero. The resonating structure of the carbon dioxide molecule equivalent with resonance bond energy 33 kcal/mole.
Structure of Boron trichloride Molecule
The boron atom is the central atom in the 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, the BCl3 molecule is a trigonal planner shape. In this molecule central atom boron with an incomplete octet. From the definition of Lewis acids bases in chemistry, BCl3 molecule affinity to accept electron pair acts as a Lewis acid by forming a coordinate covalent bond with other molecules or Lewis bases.
Structure of Methane Molecule
The methane molecule is the simplest hydrocarbon of alkane or paraffin molecule having molecular formula CH4. In methane molecule, exited carbon atom look like 2px1 2py1 2pz1 electronic configuration. On mixing these four atomic orbitals give four equivalent sp3-hybrid orbitals. These hybrid orbitals are 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 in the methane molecule.
Ammonia, Water, Hydrogen Fluoride Molecules
In chemistry, ammonia molecule nitrogen atom is the central atom, define valence shell electronic structure, 2s2 2px1 2py1 2pz1. Therefore, four atomic orbitals of nitrogen on hybridization to give four equivalent sp3-hybrid energy levels. Three of these four orbitals in ammonia molecule overlap with 1s-orbitals by uses hydrogen to form three sigma-bond and the remaining hybrid orbitals contain a lone pair of electrons acts as a Lewis base. Therefore ammonia molecule looks like the distorted tetrahedral shape with the bond angle 107.5º.
For example, water and hydrogen fluoride common list of molecules form 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. To explanation molecular structure, these two molecules form by two or three lone pairs respectively define the distorted tetrahedral shape in each molecule.