Bond Parameters:Bond Length,Bond enthalpy, Resonance Structure,Polar Bond, Dipole Moment
Bond Parameters
Bond length
It the distance between centers of nuclei of two atoms. It is expressed in Angstrom
The bond length in a covalent bond AB is given by =rA + rB
Here rA and rB are covalent radii of Atom A and B
Bond angle
It is defined as the angle between the lines representing the orbital’s containing the bonding electrons.
More the size of central atom lesser the bond angle Eg: NH3 is having higher bond angle than BH3 (Phosphine)
Bond Enthalpy
It is defined as energy required to break one mole of bonds of a particular type to separate them into gaseous atoms. It is also known as bond dissociation enthalpy. Example
Bond dissociation enthalpy for H2 is 435.8 KJ/ mole.
Order of Bond dissociation enthalpy
C ≡ C > C = C > C - C
More the multiplicity of bond, more the bond enthalpy.
In poly atomic molecules the term average bond enthalpy is used.
Bond Order
Bond Order is given by the number of bonds between the two atoms in a molecule. The bond order, for example in H2 (with a single shared electron pair), in O2 (with two shared electron pairs) and in N2 (with three shared electron pairs) is 1,2,3 respectively. Similarly in CO (three shared electron pairs between C and O) the bond order is 3
Resonance & Resonating Structures
When more than one structure with similar energy, position of nuclei, bonding & non – bonding electron pairs are possible, these structures are known as resonating structures of molecule & phenomena is called resonance.
These structures are taken as the canonical structures of the hybrid which describes the molecule accurately. Example
O3 structure:
Structure I and II are called canonical structures and Structure III is called the resonance hybrid structure which is more accurate structure and it has been proved experimentally
Resonating Structure of H NO2
H – O – N = O H – O = N – O
Resonating Structure of CO2
O = C=O O – C ≡ O O ≡ C - O Important points
1)The cannonical forms have no real existence.
2) The molecule does not exist for a certain fraction of time in one cannonical form and for other fractions of time in other cannonical forms.
3) There is no such equilibrium between the cannonical forms as we have between tautomeric forms (keto and enol) in tautomerism.
4) The molecule as such has a single structure which is the resonance hybrid of the cannonical forms and which cannot as such be depicted by a single Lewis structure.
Polarity of Bond
Non – Polar Covalent bond
When covalent bond is formed between atoms of same electronegativities, there is no shifting of shared electron and the bond is known as non – polar bond.
Example
H2 Polar Covalent bond
When covalent bond is formed between atoms of different electronegativities shared electron pair get shifted towards more electronegative atom as a result partial charge appears. Hence, pond is polar bond.
Example HF Note
Greater the difference in electronegativities of bond atoms, greater the polarity of bond.
Example
H F > H Cl > H Br as the Electronegativity order is F > Cl > Br
Because of this polarisation, the molecule possesses the dipole moment which can be defined as the product of the magnitude of the charge and the distance between the centers of positive and negative charge. Dipole Moment
Here, µ = q × l
Where, q = Charge of atom
l = Distance of Separation between atoms
a) For polar molecules dipole moment exists.
b) Dipole movement is defined as the product of the magnitude of positive or negative charge & the distance between the charges.
c) Unit is D (Debye)
1D = 3.3
d) Dipole movement is represented like this:
e) It is a vector quantity.
f) The arrow of dipole movement is away from lone pair.
g) For Symmetrical molecules, net dipole moment is O. like Be Cl2, B73, CH4, P Cl5 etc. Applications of Dipole Moment
a) To determine the polarity of molecules.
b) In finding the shapes of molecules. (The molecules with zero dipole moment will be linear or symmetrical)
