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| Valence bond theory | ||||||||||
A covalent bond is formed by the overlapping of partially filled orbitals
of two atoms.
Overlapping orbitals must have electrons with opposite spin.
Atoms involved in bond formation should have unpaired electrons.
The number of covalent bonds formed by an atom would be equal to the number
of half filled orbital.
Resulting molecular orbital is obtained by the combination of the two wave
functions (AOs) of two unpaired electrons.
Atoms which are involved in bond formation maintain their identity. |
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Molecular orbital theory |
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A covalent bond is formed by the overlapping of atomic orbitals which form
molecular orbital.
Bonding electrons occupy molecular orbital not atomic orbital.
An electron in a molecular orbital is polycentric because it is influenced
by more than one nuclei.
Formation of molecular orbital is based on the linear combination of atomic
orbitals (LCAO).
There are two kinds of molecular orbitals: 1. BONDING MOLECULAR ORBITAL 2. ANTI-BONDING MOLECULAR ORBITAL |
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Sigma
bond
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| 1.
Sigma bond is formed by the linear or head to head or end on overlapping
of orbitals. 2. Sigma bonds are the strong bonds due to maximum overlapping of orbitals. 3. Electron density is maximum around the bond axis. 4. Compounds having sigma bonds are stable and hence less reactive. 5. First bond formed between two atoms is always a sigma bond. 6. s-s overlap, s-p overlap and p-p overlap give rise to sigma bond. |
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| Examples of Sigma bond | ||||||||||
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s-s
overlap in H2 molecule
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| Hydrogen molecule consists of two H-atoms. Each atom contains one electron in 1s-orbital. E.C = 1s1 | ||||||||||
| According
to molecular orbital theory two 1s1 orbitals of two H-atoms
overlap linearly to produce two types of molecular orbitals. 1. Bonding molecular orbital (s-orbital) 2. Anti-bonding molecular orbital (s*-orbital) Bonding molecular orbital (s-orbital) has lower energy and results in the covalent bond formation which is a sigma bond between two H-atoms while the anti-bonding molecular orbital (s*-orbital) possess high energy remains unoccupied. |
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s-p
overlap in HF molecule
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s-p
type overlap occurs in HF molecule when one 1s-orbital
of hydrogen atom overlaps 2p-orbital of fluorine to form two
types of molecular orbitals. |
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| p-p overlap in HF molecule | ||||||||||
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p-p
type overlap occurs in F2
molecule when one 2pz-orbital
of one fluorine atom overlaps 2pz-orbital
of other fluorine atom form two
types of molecular orbitals.
1. Bonding molecular orbital (s-orbital) 2. Anti-bonding molecular orbital (s*-orbital) Bonding molecular orbital (s-orbital) has lower energy and results in the covalent bond formation (sigma bond) between two F-atoms while the anti-bonding molecular orbital (s*-orbital) possess high energy remains unoccupied. |
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Strength
of sigma bond
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The relative strength of a sigma bond is related to the extent of overlap of the atomic orbitals. This is known as the 'principle of maximum overlap'. |
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| Due
to spherical charge distribution in s-orbital, generally s-s overlapping
is not so effective as s-p and p-p overlapping. Where p-orbitals have directional charge distribution and longer lobes which cause more effective overlapping. Thus s-s sigma bond is relatively weak. Order of the strength of sigma bonds is as follows: |
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Pi
bond
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| A
Pi bond is formed by the lateral or side ways or parallel overlapping of
P-orbital of the atoms which are already bonded by a sigma bond and their
axes are coplanar. This type of overlap generates two types of molecular
orbitals: (a) Pi-bonding molecular orbital (p-orbital) (b) Pi-antibonding molecular orbital (p*-orbital) A pi-bonding orbital has two regions of electron density below and above the nodal plane.The electron contained in it are called pi-bonding electrons which form the pi bond. It is not linearly symmetrical with respect to the bond axis, rather it has a nodal plane. Pi-bonds are weaker than sigma bonds. In Pi-bonds, electron density lies in the regions above and below the nuclei. Compounds having pi bonds are more reactive. Pi bond is formed when two atoms already bonded by a sigma bond. |
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us:
info@citycollegiate.com |
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