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Wednesday, 21 October 2015

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 5

FSc Notes Chemistry Part 1 Chapter 6 Chemical Bonding Lecture 5

Valence Bond Theory ( VBT ):

The VSEPR theory successfully explains and predicts the geometry of molecule but totally fails in explaining that how atoms share electrons in the valence shell to form covalent bonds which result in the formation of molecule. So valence bond theory explains the sharing of electron by bonding atoms to form covalent bonds, successfully VBT is based on quantum mechanics. According to VBT, the bonding atoms overlap their half filled orbital’s ( orbital’s having one electron ) in such a way that both the unpaired electrons each present in the half filled orbital of each atom ) get paired in the overlapped region with an opposite spin resulting in the formation of a covalent bond. Greater the overlapped region, stronger will be the bond formed and vice versa.

Postulates (Main Points):
Covalent bond is formed by the overlap o f the filled orbital’s of the bonding atoms where they share their electrons. During the formation of a covalent bond only those orbital’s take part which have one electron in them i.e. which are half filled. The orbital’s containing a pair of electrons ( i.e. which are complete ) don’t undergo overlapping and hence they don’t participate in bond formation. The mutual sharing of electron b/w atoms ( as a result of overlapping ) occurs in two ways and thus producing two types of covalent bond i.e. Sigma (s) bond & (b) Pi bond.

Sigma Bond ( S )
When two partially filled ( half filled ) atomic orbital’s overlap in such a way that the probability of finding the electrons is maximum around the line joining the two nuclei, then it is called a sigma bond.
Or
All such bonds in which the region of highest density is around the bond axis, are known as sigma covalent bonds.
  • All single covalent bonds are sigma bond.
  • As sigma bond is always formed by the linear overlapped of atomic orbital’s, therefore maximum portion of atomic orbital’s comes in the overlapped region and that is why, sigma bond is very strong and stable. In other words we can say that large amount of energy evolves during the formation of a sigma bond hence it is very stable.

Pi Bond:
The bond formed by the side wise or parallel overlap of two half filled co-planner "P" orbital’s in such a way that the probability of finding the electrons is maximum, perpendicular to the line joining the two nuclei is known as Pi bond.
Or
The bond which is formed by the side wise or parallel overlap of half filled P—orbital’s, is known as Pi bond.

The overlap of atomic orbital’s of the bonding atoms may be S—S overlap, P—P overlap or S—P overlap.

i. S—S overlap
When half filled is orbital of one atom overlaps with half filled s orbital of other atom, then such an overlap is known as S—S overlap and always results in the formation of a sigma bond.
For example:
H2 Molecule has two H atoms. Each H atom has a is half filled orbitals. According to valence bond theory both the H atoms overlap their half filled S orbitals resulting in the formation of a sigma covalent bond b/w H atoms and thus H2 is formed.

ii. S—P overlap:
When half filled S—orbital of one bonding atom overlaps with half filled P orbital of other bonding atom them such an overlap is known as S—P overlapped. As a result of S—P overlap, always a stigma covalent bond is formed.
For example : In HCl molecule, is ( half filled ) orbital of H atom overlaps with the 3Pz ( half filled ) orbital of Cl atom, to form a sigma covalent bond B/W H and Cl atoms and thus HCL is produced, ie
1H = 1S1 (1)
17 CL = IS2 , 2S2 , 2P6 , 3S2 , 3P5
Ie
H + Cl ----------H—Cl

iii. P—P Overlap:
When a “ P” orbital ( half filled ) of one atom overlaps with the “P” ( half filled ) orbital of other atom of form a covalent bond, then such an overlap is known as P—P overlap. As “ P “ orbit has 3 orbital’s ie Px, Py & Pz, therefore P—P orbital, overlap may be:
Linear overlap & (B) Parallel overlap.
Linear Overlap: When two P orbitals of the two bonding atoms approach each other with their heads directly towards one another, such a P—P overlap is known as linear P—P overlap and results in the formation of a sigma bond eg: in case of Cl2, F2, Br2 etc.

Parallel Overlap:
When two “ P “ orbitals overlap side wisely, such an overlap is known as parallel P—P overlap, resulting in the formation of a Pi bond. Such an overlap can take place only, if a P—P linear overlap has already taken place, b/c the sigma bond brings the atoms closer to each other making an opportunity for the side-wise overlap of the remaining P orbitals ( half filled ) eg in O2 molecule, each “ O” atom has two half filled “ P” orbitals. One each of them the remaining half filled “ P “ orbitals undergo parallel overlap to form a bond.

Drawbacks of VBT:

Some serious drawbacks of VBT are:
  • It doesn’t give an idea about the geometry of molecule.
  • It states that only the half filled orbitals get disturbed during bond formation because they undergo overlapping while all other orbitals ( which are complete ) remain undisturbed, which is impossible. It cannot explain the para-magnetic character of oxygen molecule. All these drawbacks are removed by MOT, so we say that MOT is superior to VBT.


Written by: Asad Hussain

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