Summary

Valence bond theory assumes that the electrons in a molecule occupy atomic orbitals of the individual atoms, a covalent bond forms as a result of electron sharing between the bonded atoms. A single covalent bond results when two atoms share a pair of electrons. Sometimes atoms share two or three pairs of electrons. trons,in these cases we say that a double or triple bond, respectively, has been formed. The concept of atomic orbital overlap is applied to molecules, Valence bond theory states that a stable molecule forms from reacting atoms when the potential energy of the system has decreased to a minimum.

In hybrid orbital theory, valence atomic orbitals on the same atom that are fairly close in energy, for ex.ample, s and p orbitals,can be combined to form hybrid atomic orbitals, The number of hybridized orbitals is equal to the number of pure atomic orbitals combined  There are two types of equivalent and nonequivalent hybridization for sp3 - hybridized atom. Hybridization type of the central atom is used to explain molecular geometries.

The valence shell electron pair repulsion model is based on the idea that both bonding and nonbonding electron pairs in valence shell of the central atom repel each other and tend to stay as far apart as possible.Molecular geometry can be predicted and the hybridization type of the central atom can be deduced from the number of bonding electron and lone pairs . lt is the first necessary to calculate electron pairs in valence shell of the central atom.

Molecular orbital theory describes covalent bonds in terms of molecular orbital, Molecular orbitals are wavefunctions for electrons in molecules. Overlapping atomic orbitals can give bonding and antibonding molecular orbital. Molecular orbital energy level diagrams are two cases for the second period homonuclear diatomic molecules. Electrons are assigned to molecular orbital . In accordance with the order of molecular orbital energy level, the Pauli exclusion principle and Hund’s rule, we can write the electronic configuration of molecules and predict the magnetic properties and bond orders of second-period diatomic molecules. The bond order (BO) is defined as half the number of net bonding electrons. Molecular species are stable only if the bond order is greater than 0 . The larger the bond order , the shorter and stronger the bond.

Intermolecular forces act between molecules; between molecules and ions or between non-charged or charged groups on the side chain of polymers in living systems. Weak attractive forces between uncharged atoms or molecules are known as van der Waals forces. The term “van der Waals forces" refers to the total effect of dipole-dipole , dipole-induced dipole, and dispersion forces.. The magnitude of dispersion force in.creases with an increase in the number of electrons, and therefore substances of higher molecular mass generally have higher melting and boiling points. lf two substances have approximately the same molecular mass,and one is polar while the other is not, the polar substance will have the higher melting and boiling point.

Hydrogen bonding is a relatively strong dipole-dipole force that acts between a polar bond containing a hydrogen atom and bonded strong electronegative atoms, In particular, hydrogen bonding has profound effects on the structure of proteins and DNA (deoxyribonucleic acid) , the basic genetic material.