1. Mr. Sayad Imran, Asst. Professor Y B C C P- A
Bonding Parameters
Mr. Sayad Imran, Asst. Professor
Y B C C P- A
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4. Bonding Parameters Bond Length Bond Order Bond Energy Bond Angle
Steric Effect
Resonance
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5. It is expressed in Angstrom units (Å) or picometers (pm).
Bond Length
Bond length is the average distance between the centers of the nuclei of two bonded atoms in a molecule.
It is expressed in Angstrom units (Å) or picometers (pm).
1Å = 10-10m and 1pm = m.
It is determined by X-rays diffraction, electron diffraction and other spectroscopic methods.
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6. Vibrational motions in general diatomic and triatomic molecules.
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7. Covalent radius and Bond length
One half the equilibrium distance between atomic nucleus of two covalent bonded atoms of same kind is called covalent radius.
The sum of covalent radii of two atoms is usually the bond length.
For example, the covalent radii of H and C are 37 and 77 pm respectively. The C-H bond is thus (37+77) 114 pm.
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8. Bond length and covalent radius.
Internuclear distance
(bond length)
Covalent radius
72 pm
Internuclear distance
(bond length)
Covalent radius
114 pm
Internuclear distance
(bond length)
Covalent radius
100 pm
Internuclear distance
(bond length)
Covalent radius
133 pm
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10. Factors affecting Bond length
Bond Order
Hybridization state
Size of the atom
Electronegativity
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11. Bond length decreases with increase in bond order.
The bond order is the number of electron pairs being shared by a given pair of atoms.
A single bond consists of one bonding pair and has a bond order of 1.
For a given pair of atoms, a higher bond order results in a shorter bond length and higher bond energy
C≡C bond length is shorter than C=C bond which in turn is shorter than C-C.
Similarly, N≡N bond length is shorter than N=N bond which in turn is shorter than N-N and O=O > O-O.
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12. The Relation of Bond Order, Bond Length, and Bond Energy
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13. nucleus than do p orbitals.
Hybridization State
The bond lengths are also affected by the hybridization states of the carbon atoms involved.
● The greater the s orbital character in one or both atoms, the shorter is the bond length.
This is because s orbitals are spherical and have more electron density closer to the
nucleus than do p orbitals.
● The greater the p orbital character in one or both atoms, the longer is the bond length.
This is because p orbitals are lobe-shaped with electron density extending away
from the nucleus.
Hybridization
% S
% P sp 50
sp2 33 67
sp3 25 75
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14. C—F (142 pm) < C—Cl (177 pm) < C—Br (191 pm) < C—I (213 pm).
Size of the atom
The bond length increases with increase in the size of the atom i.e. atomic number
H—H < C—C < I—I
This is because as the atomic number increase, the distance of the electrons from the nucleus increases successively with the addition of a new shell. Therefore the average distance between the bonding nuclei (bond length) increases.
C—F (142 pm) < C—Cl (177 pm) < C—Br (191 pm) < C—I (213 pm).
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15. Electronegativity
The order of electronegativity of hybrid orbitals is: sp > sp2 > sp3 i.e., the electronegativity of carbon is maximum in the sp-hybridized state and minimum in sp3 -hybridized state
The a bond formed with a more electronegative atom will be shorter than that formed with a less electronegative atom.
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16. Bond Energy (Bond Dissociation Energy)
The minimum amount of energy required to break a bond is called bond dissociation energy or simply bond energy.
It gives us information about the strength of a bonding interaction.
It is expressed in terms of kJ mol-1 or Kcal/mole.
When a bond is formed between the atoms, energy is released.
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17. Bond dissociation energy depends upon: Size of the bonded atoms
The smaller the size of the bonded atoms, the stronger is the bond and larger is the value of bond dissociation energy.
For example, the bond dissociation energy of H-H bond in hydrogen molecules is 433 kJ mol-1. This is larger than the bond dissociation energy of Cl-Cl in Cl2, which is kJ mol-1.
Bond length
Shorter the bond length, larger is the value of bond energy.
For example, C-C bond length (154 pm) is larger than C=C bond length (134 pm). Consequently, the dissociation energy of C-C bond (348 kJ mol-1) is smaller than that of C=C bond (619 kJ mol-1).
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18. Important features of bond energy
The magnitude of the bond energy depends on the type of bonding.
Most of the covalent bonds have energy between 50 to 100 kcal mol–1.
Strength of sigma bond is more than that of a π-bond.
A double bond in a diatomic molecules has a higher bond energy than a single bond
and a triple bond has a higher bond energy than a double bond between the same atoms.
C ≡ C > C = C > C – C
The magnitude of the bond energy depends on the size of the atoms forming the
bond, i.e. bond length. Shorter the bond length, higher is the bond energy.
Resonance in the molecule affects the bond energy.
The bond energy decreases with increase in number of lone pairs on the bonded
atom, due to electrostatic repulsion of lone pairs of electrons of the two bonded
atoms.
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19. Homolytic and heterolytic fission involve different amounts of energies.
Generally the values are low for homolytic fission of the bond in comparison to heterolytic fission.
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20. Yellow fill circle indicate center atom
Bond Angle
Lewis structures of molecules are two dimensional, giving only the image of bonding in the molecule. But, all molecules containing three or more atoms are three dimensional in nature.
The shape of a particular molecule is determined by the specific arrangement of atoms in it and the bond angles.
Polyatomic molecule contain at least two or more atom covalently bonded with central atom.
Yellow fill circle indicate center atom
The angle between any two covalent bond axes
at central atom in a molecule is Bond angle
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22. Bond angle measured by X-ray diffraction and spectroscopic methods.
Because of the constant atomic vibrations, the bond angles thus measured are really average bond angles.
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23. Factor affecting Bond Angle Electronegativity of central atom
The bond angle of a molecule having the same terminal atom decreases with decreasing electronegativity.
H2O < H2S < H2Se [Hyderogen selenide]
104028’ ’
Effect of Electronegativity or Size of terminal atom
Bind angle increases as Electronegativity of terminal atom decreases.
Bind angle increases as size of terminal atom increases.
PF3 PCl3 PBr3 PI3
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24. Number of regions of high electron density around central atom
Molecular shapes and bond angles
Number of regions of high electron density around central atom
Arrangement of regions of high electron density in space
Predicted bond angles
Example
Geometry of molecule 4
tetrahedral
109.5°
CH4, methane
NH3, ammonia
pyramidal
H2O, water
bent 3
trigonal planar
120°
H2CO, formaldehyde
C2H4, ethylene
planar
SO2, sulfur dioxide 2
linear
180°
CO2, carbon dioxide
C2H2, acetylene
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25. Steric Effect
The bulk or volume of an atom or group of atom on the reacting part of an organic species and their special arrangement has varied type of effect on stability, reactivity(acidity, basicity), rate of reaction, stereochemistry and yield of product these effect may be called Steric Effect.
Type of Steric Effect
i. Steric Strain
ii. Steric Acceleration please refer Sachin Ghosh
iii. Steric Retardation Page no
iv. Steric Effect and electron availability }
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