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Physical Inorganic Chemistry :

THE PRINCIPLES THE PRINCIPLES OF BOND OF BOND

STRENGTH STRENGTH Part-1Part-1

A Background OverviewA Background Overview

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CHEMICAL BONDING

Electrostatic Approach

Covalent Approach

Ionic Bond Ionic Bond

Covalent Bond Covalent Bond

Metalic BondMetalic Bond

Hydrogen Bond Hydrogen Bond

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Normal Covalent Normal Covalent BondBond

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CHCH44

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Axial Axial OverlapingOverlaping

- bonding- bonding

CHCH44 : :

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sp3 Hybrid Orbital

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CC22HH4 4 :: - - bondingbondingLateral Lateral

OverlapingOverlaping

Axial OverlapingAxial Overlaping

- bonding- bonding

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spsp2 2 Orbital Orbital HybridHybrid

Orbital Orbital ppzz

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NHNH33

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NHNH3 3 ::

Non Bonding ElectronsNon Bonding Electrons

Lone Pair ElectronsLone Pair Electrons

Free ElectronsFree Electrons

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HH22OO

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HH22OO ::

Non Bonding Non Bonding ElectronsElectrons

Lone Pair ElectronsLone Pair Electrons

Free ElectronsFree Electrons

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Atomic ChargeAtomic Charge

++ --

NHNH33

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Atomic ChargeAtomic Charge

--++

HH22OO

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Formal ChargeFormal Charge

Definition :

Formal charge may be regarded as the electrical charge that an atom in a molecule would have if all of the atoms had the same electronegativity (all chemical bonds involved, are assumed to have 0 % ionic character) .

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Oxidation NumberOxidation Number

Definition :

Oxidation number may be regarded as the electrical charge that an atom in a molecule would have if all chemical bonds involved, are assumed to have 100 % ionic character.

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Both “Formal Charge” and “Oxidation Number” are just arbitrary quantities.

The appropriate assignment of both formal charge and oxidation number is a matter of convenience (or inconvenience!).

WARNING ! ! !WARNING ! ! !

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The Principles :The Principles :

• Covalent bonds between small Covalent bonds between small atoms tend to be stronger than atoms tend to be stronger than covalent bonds between larger covalent bonds between larger atoms atoms

• Covalent bonds between atoms Covalent bonds between atoms tend to be weaker, the greater tend to be weaker, the greater the difference in principal the difference in principal quantum number of the valence quantum number of the valence shell of the two elementsshell of the two elements

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• Covalent bond strength tends to Covalent bond strength tends to be greater, the greater the bond be greater, the greater the bond polarity.polarity.

One explanation of this tendency has been One explanation of this tendency has been provided by Pauling, who considered the extra provided by Pauling, who considered the extra energy resulting from bond polarity to be derived energy resulting from bond polarity to be derived from supplementing the covalent bond energy from supplementing the covalent bond energy with the ionic attraction between unlike charges. with the ionic attraction between unlike charges. Pauling used this extra energy in establishing his Pauling used this extra energy in establishing his well-known scale of electronegativity.well-known scale of electronegativity.

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• Covalent bonds between atoms Covalent bonds between atoms each bearing partial positive each bearing partial positive charge tend to be less stablecharge tend to be less stable

• Covalent bonds having partial Covalent bonds having partial multi-plicity as a result of multi-plicity as a result of resonance are more stable than resonance are more stable than single bonds or than the single bonds or than the equivalent of partial multi-plicity equivalent of partial multi-plicity in separate single and integral in separate single and integral multiple bonds multiple bonds

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PentadienePentadiene C C55HH8 8 ::

Delocalized Delocalized Bonding Bonding

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Localized Localized BondingBonding

PentadienePentadiene C C55HH8 8 ::