Plz explain VB theory & VSEPR theory.(short and sweet-ly)?

Answer 1

Molecules exist in different shapes. Many of the physical & chemical prpoerties of molecules arise due to the different shapes.

Some of the common geometrical shapes are trigonal, planar, tetra hedral etc.The VSEPR theory provides a simple treatment for predicting the shapes of polyatomic molecules. The theory was originally proposed by Sigdwick and Powell in 1940.

In case of molecules with central atom having one or more lone pairs VSEPR treatment is as follows: In these type of molecules, both lone and bonded pairs of electrons are present. The lone pairs are localised on the central atom, & bonded pairs are shared between the two atoms. Consequently, the lone pair of electrons occupy more space as compared to the bonding pair. This causes great repulsion between lone pair of electrons.

These repulsion effectscause deviations in the predicted bond angles in molecules.

The descending order of repulsion interaction is lp - lp > lp - bp > bp - bp

lp - lone pair
bp - bonded pair

Answer 2

Vsepr And Valence Bond Theory

Answer 3

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Answer 4

Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond. Due to the overlapping, it is most probable that electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules.

The overlapping atomic orbitals can be of different types. There are two different types of overlapping orbitals: sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap co-axially(i.e head-on overlap). Pi bonds occur when two orbitals overlap laterally(i.e. the side-to-side overlap of p-orbitals). For example, a bond between two s-orbital electrons is a sigma bond, because two spheres are always coaxial. In terms of bond order, single bonds consist of one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds consist of one sigma bond and two pi bonds.

However, the atomic orbitals for bonding may not be "pure" atomic orbitals. Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an atomic orbital with the proper character for the bonding is called hybridization (also spelled hybridisation). Hybridization can only occur when electrons need to be promoted to the next energy level.

VB theory today

Valence bond theory now complements Molecular Orbital Theory (MO theory), which does not adhere to the VB idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule. Compared to MO theory, VB theory performs poorly when predicting magnetic properties and aromatic properties of molecules. The underlying mathematics are also more complicated limiting VB treatment to relatively small molecules.

More recently, several groups have developed what is often called modern valence bond theory. This replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over all basis functions in the molecule. The resulting energies are more competitive with energies where electron correlation is introduced based on a Hartree-Fock reference wavefunction.

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VSEPR theory is based on the idea that the geometry of a molecule or polyatomic ion is determined primarily by repulsion among the pairs of electrons associated with a central atom. The pairs of electrons may be bonding or nonbonding (also called lone pairs). Only valence electrons of the central atom influence the molecular shape in a meaningful way.

Three types of repulsion take place between the electrons of a molecule:

* The lone pair-lone pair repulsion
* The lone pair-bonding pair repulsion between the atoms
* The bonding pair-bonding pair repulsion.

A molecule must avoid these repulsions to remain/stay stable. The theory states that; repulsion becomes zero at ~115-120°. When repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred.

The lone pair-lone pair (lp-lp) repulsion is considered to be stronger than the lone pair-bonding pair (lp-bp) repulsion, which in turn is stronger than the bonding pair-bonding pair (bp-bp) repulsion. Hence, the weaker bp-bp repulsion is preferred over the lp-lp or lp-bp repulsion.

Larger molecules which fail to even maintain 90° between their electron pairs prefer to lie in more than one plane.

Examples

The methane molecule (CH4) is tetrahedral because there are four pairs of electrons. The four hydrogen atoms are positioned at the vertices of a tetrahedron, and the bond angle is cos-1(-1/3) ≈ 109°28'. This is referred to as an AX4 type of molecule. As mentioned above, A represents the central atom and X represents all of the outer atoms.

The ammonia molecule (NH3) has three pairs of electrons involved in bonding, but there is a lone pair of electrons on the nitrogen atom. It is not bonded with another atom; however, it influences the overall shape through repulsions. As in methane above, there are four regions of electron density. Therefore, the overall orientation of the regions of electron density is tetrahedral. On the other hand, there are only three outer atoms. This is referred to as an AX3E type molecule because the lone pair is represented by an E. The overall shape of the molecule is a trigonal pyramid because the lone pair is not "visible." The shape of a molecule is found from the relationship of the atoms even though it can be influenced by lone pairs of electrons.

In fact, a steric number of seven is possible, but it occurs in uncommon compounds such as xenon hexafluoride. The base geometry for this is pentagonal bipyramidal. The trend for this configuration is the same as for the octahedral configuration: the first nonbonding electron domain would be in the axial position, making the actual molecular geometry pentagonal pyramidal.

Answer 5

VB T MEANS VALENCE BOND THEORY.
VALENCE SHELL ELECTRON PAIR REPULSION THERY
DUMMY BUYE