Table of contents | |
Valence Bond Theory | |
Hybridization Of Atomic Orbitals | |
Orbital Analysis of Hybridization | |
Some Solved Questions |
According to this theory, a covalent bond is formed by the overlapping of two atomic orbitals of proper energy and proper symmetry. This theory was given by Hietler and London and extended by Pauling and Slater.
For a covalent bond to form, two atoms must come closer to each other so that orbital of one atom overlaps with the other.
The extent of overlapping depends upon:
(a) nature of orbitals involved in overlapping.
(b) nature of overlapping.
(σ-Bond is formed by the axial overlapping of atomic orbitals)
Formation of sigma bond
(p-bond is formed by the sideways overlapping of atomic orbitals)
(a) (i) Intermolecular distance (x) repulsion P.E. stability
(ii) Also in an s-orbital, ψ is positive throughout but in p-orbital it is positive and negative
∴ Total overlapping will zero
not allowed.
Limitations of Valence Bond Theory: This theory could not explain the following facts:
(i) It fails to explain the paramagnetic behaviour of O2 molecules.
(ii) It fails to explain the bonding in electron deficient compounds as well as in metals and intermetallic compounds.
(iii) It lacks in mathematical explanation of most chemical species, as the valence bond concept is very close to the classical chemical picture. In classical chemical picture, a bond is denoted by a line whereas in valence bond theory, the electron pair plays the same role.
The concept of hybridization lies in its origin in VBT.
In VBT, the ideal of hybridization was required to explain the faces:
(i) the number of bonds formed,
(ii) the equivalence of the bonds in some cases,
(iii) the stereochemistry of the molecules,
(iv) the better overlapping of the orbitals. Formation of sp2 hybridization
Table of hybridisation
If all the hybrid orbitals after hybridization are equivalent to one another in all the manners then it is called equivalent hybrid orbitals.
For example: in the case of sp3, s + px + py + pz produce four equivalent orbitals.
In sp3d, the five hybrid orbitals are not equivalent. The three hybrid orbitals arising from s, px, and py are equivalent lying at an angle 120º in the xy-plane. The other two hybrid orbitals made of pz and d z2 are projected mutually at 180º, and perpendicular to the xy-plane.
The sp3d3 hybridization (pentagonal bipyramid) can also be analyzed in the same way giving rise to two different sets of hybrid orbitals: leading to a pentagon in the xy-plane; (pz + ) leading to a linear segment perpendicular to the pentagon.
According to the rule whenever P and onwards elements or attach to H atom, in such compounds lone pair generally do not participate in hybridization.
Example: PH3; H2Se, AsH3, SbH3, BiH3, H2S, H2Te, etc.
(i) Determination of Lewis basic strength: Hybrid orbitals shows better basic character than unhybridized orbitals as they have more directional character.
Example:
NH3 is a better Lewis base than PH3 because the lone pair in NH3 is present in sp3 hybridized state while in PH3 s orbital is unhybridized.
Similarly:
(a) NH4+ forms easily but not PH4+
(b) NH3 readily reacts with H2O but not PH3
(c) NH3 is more soluble in water but not PH3
(d) Aqueous solution of NH3 has a greater pH value than aqueous solution of PH3.
(ii) Effect on bond angle: The compounds in which hybridization does not take place possess a bond angle of nearly to 90º.
(iii) Existence and non-existence of molecules on the basis of hybridization: For hybridization, the energy difference between the ground state and the excited state must not be very high.
Example: CH4 exists but PH5 does not this is because in CH4, there is an s-p transition taking place, and as the energy difference between s and p orbital is very less whereas in PH5, there is need to be an s-d transition, which is not favorable.
Lesser is the excitation energy better is the hybridization. Excitation can be made easier in 2 ways:
(a) By supplying energy
(b) By reducing the energy gap between orbitals or orbital contraction. This can be done by attaching any electronegative element to the central atom.
For example: In the case of PF3, F being highly electronegative contracts the d orbital of Phosphorus thereby resulting in hybridization while in case of PH3 there is no hybridization. Similarly, in case of PCl3 orbital contraction occurs due to chlorine atoms.
Similarly,
1. PCl5 exist but not PH5
2. 3rd period and onwards elements do not show maximum covalency when attached to lesser electronegative elements like H.
3. Xe forms compounds with F, O like atoms but does not form compounds with H.
knowing the value of i, we can determine hybridization which is equal to spi
Illustration: Determines character in lone pair for PH3 having bond angle 94º.
Solution:
S = 0.06
% of s character in lone pair = 100 – (3 × 6) = 82%
a) may be or may not be related
b) is independent on
c) is dependent on
d) is not related to
Answer: c
Explanation: As per the concept of valence bond theory, the partial merging of atomic orbitals id knowns as overlapping. The extent of overlapping is directly proportional to the strength of the covalent bond, i.e. it is dependent.
Q.2. Which type of bond is present between hydrogens in hydrogen molecule?
a) Sigma bond
b) Pi bond
c) Ionic bond
d) Metallic bond
Answer: a
Explanation: The head-on or end-to-end type of overlapping is present in the sigma bond. A sigma bond is a type of covalent bond. It may also be called an axial overlap. In case of the hydrogen molecule, its s-s overlapping.
Q.3. The pi-bond involves __________
a) axial overlapping
b) side-wise overlapping
c) end to end type of overlapping
d) head-on overlapping
Answer: b
Explanation: A pi-bond is a type of covalent bond in which the internuclear axes of the atoms are parallel to each other and for side-wise overlapping. The bond formed here is perpendicular to the internuclear axes.
Q.4. A __________ overlap doesn’t result in the formation of a bond.
a) positive
b) negative
c) zero
d) rational
Answer: c
Explanation: Zero overlap means that the orbitals don’t overlap at all. When there is no overlapping the bond formation doesn’t occur. As we all know that the extent of overlapping is dependent on the strength of the bond.
Q.5. A positive overlap is same as ________
a) out-phase overlap
b) negative overlap
c) zero overlap
d) in-phase overlap
Answer: d
Explanation: A positive overlap results in bond formation. When 2 p-orbitals are in phase, both the positive lobes overlap, thus creating a positive overlap and result in the bond formation, thus it is called in-phase overlap.
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1. What is Valence Bond Theory? |
2. What is the concept of Hybridization of Atomic Orbitals? |
3. How is Orbital Analysis used in Hybridization? |
4. Can you provide some examples of Hybridization and its application in chemical bonding? |
5. Are there any solved questions related to Valence Bond Theory and Hybridization? |
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