Valence bond (VB) theory describe that atoms in a covalent bond share the electron density by overlapping their valence atomic orbitals. The valence bond approach takes consideration of overlapping atomic orbitals (AO) of the participating atoms to form a chemical bond. Quantum mechanics describe the chemical bond in VB by applying the principle of orbital merging that is essential and to achieve the merging; two electrons have to pair their spins (Shurki et al., 2015) . A molecular orbital (MO) approach is an alternative approach which proposes that electrons are part of the entire molecule. Hybridized orbitals are formed when AO combine with a proper character for bonding.
The fundamental principle of both VB theory and MO theory is that both involve the distribution of electrons and energy of overlapping orbitals should be comparable and have similar symmetry. The significant differences between the two are that in VB theory, the bonds are localized to two atoms compared to bond in MO theory where bonds are confined in two atoms as well as molecules. Additionally, resonances play an essential role in VB theory while in MO theory, there is no place for resonance (Shurki et al., 2015) . For instance, in VB BeH 2 two equivalent orbitals can be generated by combining 2s orbital of beryllium and any other of the three to degenerate 2p orbitals to form a linear structure. In contrast, MO theory proves that Be 2 does not exist because it lacks half-filled orbital and therefore overlapping is not possible. The oxygen atom is sp3 hybridized in the H 2 O molecule, single pairs occupy two hybrid orbitals, and two are used for bonding with a hydrogen atom. Similarly, MO theory considers electrons as delocalized in the entire molecule and the 2s orbital in oxygen is mixed with hydrogen orbitals (Shurki et al., 2015) .
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The number of new hybrid orbitals should be equal to the number of atoms in non-bonded pairs of electrons surrounding the central atom. The valence shell electron-pair repulsion model (VESPR) is devised on account of molecular shapes (Dean, Ewan, and Mclndoe, 2016) . Through VESPR model atoms and electrons, pairs are arranged to minimize repulsion of atoms and electrons. Resonance structure describes the delocalization of electrons within molecules and illustrates areas of higher electron densities. The Lewis theory focuses on the valence electrons of the outermost energy level (Weinhold & Klein, 2014) . Lewis diagrams pictures valence electrons shared during covalent bonding. The covalent bond formed between atoms is used to satisfy the octet rule. The Octet rule describes molecular structure whereby a molecular lose, share, or gain valence electrons until they share or attain eight electrons. The polarity of bonds is due to the existence of both ionic and covalent bonds to represent an ideal situation. The Hund’s rule describes that every orbital in a subshell is individually occupied with one electron before any orbital is doubly occupied, and all electrons have the same spin in singly occupied orbitals (Christensen et al., 2015).
Reference
Christensen, M. H., Kang, J., Andersen, B. M., Eremin, I., & Fernandes, R. M. (2015). Spin reorientation drove by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides. Physical Review B , 92 (21), 214509.
Dean, N. L., Ewan, C., & McIndoe, J. S. (2016). Applying Hand-Held 3D Printing Technology to the Teaching of VSEPR Theory.
Shurki, A., Derat, E., Barrozo, A., & Kamerlin, S. C. L. (2015). How valence bond theory can aid you to understand your (bio) chemical reaction. Chemical Society Reviews , 44 (5), 1037-1052.
Weinhold, F., & Klein, R. A. (2014). What is a hydrogen bond? Resonance covalency in the supramolecular domain. Chemistry Education Research and Practice , 15 (3), 276-285.
