Concept Of Valence Bond Theory
The overlapping of two half-filled atomic orbitals yields a pair of electrons between both linked atoms, according to the Valence Bond Theory.
As we know, molecular modeling can be understood by considering the electron-dot model or VSEPR model. The electron-dot model gives the idea of the position of electrons in the molecule.
Moreover, the VSEPR model gives the shape of a covalent molecule that is 3-dimensional, based on the number of paired electrons occupied by the central atom.
Yet these theories are unable to explain the chemical bonding between the covalent molecule.
This approach underlies the key concepts of VB theory:
- The electron pair with opposing spins.
- The space generated by overlapping orbitals can only hold two electrons with opposite spins, according to the exclusion principle. The two 1s electrons of two H atoms, for example, occupy overlapping 1s orbitals and have opposite spins when a molecule of H2 forms.
- Bonding orbitals overlap to the maximum extent.
- The strength of the bond is determined by the nuclei's attraction to the shared electrons, hence the larger the orbital overlap, the stronger (and more stable) the bond. The extent of overlap is determined by the orbitals' shapes and orientations.
- Although an s-orbital is spherical, p and d orbitals have greater electron density in one direction than the other. Thus, the orientation of the bond with orbitals p and d will be directed to maximum overlap density.
- The 1s orbital of H, for example, overlaps the half-filled 2p orbital of F along the long axis of that orbital in the HF bond. In any other direction, there would be less overlap and consequently a weaker bond. Similarly, in F2, the two half-filled 2p orbitals interact end to end, that is, along the orbitals' long axes, to maximize overlap as shown in the figure below.
Atomic orbital hybridization
It is the process through which two or more orbitals combine to form a new orbital. We imagine the direct overlap of s and p orbitals of isolated atoms to account for bonding in simple diatomic compounds like HF.
But how can the overlap of spherical s orbitals, dumbbell-shaped p orbitals, and cloverleaf-shaped d orbitals account for the geometries of so many molecules and polyatomic ions?
These facts can be understood by the concept of hybridization which involves the mixing of atomic orbitals, and the orbitals that are derived as a result of hybridization are known as hybrid orbitals
- The number of hybrid orbitals obtained is equal to the number of atomic orbitals mixed, and the type of hybrid orbitals obtained is dependent on the atomic orbitals combined.
- Hybridization can be thought of as a process in which atomic orbitals interact, hybrid orbitals form, and electrons enter with parallel spins (Hund's rule) to form stable bonds. Hybridization, on the other hand, is a mathematically calculated consequence of quantum mechanics that explains the molecular shapes we see.
Types of Hybrid Orbitals:
- sp Hybridization
It is a type of hybridization in which one s- and one p-orbital overlap and forms two sp orbitals, while the two p-orbitals remain unhybridized. The geometry of sp hybridized molecules is linear.
- sp2 Hybridization
It is a type of hybridization in which one s- and two p-orbitals overlap and form three sp2 orbitals, and one p-orbitals remain unhybridized. The geometry of sp2 hybridized molecules is trigonal planar.
- sp3 Hybridization
It is a type of hybridization in which one s- and three p-orbitals overlap and form four sp3 hybridized orbitals. The geometry of sp3 hybridized molecules is tetrahedral.
- sp3d Hybridization
It is a type of hybridization in which one s- and three p- and one d-orbital overlap and forms five sp3d hybridized orbitals, while four d-orbitals remain unhybridized. The geometry of sp3d hybridized molecules is trigonal bipyramidal.
- sp3d2 Hybridization
It is a type of hybridization in which one s- and three p- and two d-orbitals overlap and form six sp3d2 hybridized orbitals, while three d-orbitals remain unhybridized. The geometry of sp3d2 hybridized molecules is octahedral.
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