157.
What do you mean by: (i) Bond length (ii) Bond enthalpy (iii) Bond order?
(i) Bond length: Bond length is defined as
the equilibrium internuclear separation distance of the bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length.
In the case of a covalent bond, the contribution from each atom is called covalent radius of that atom.
The bond length in a covalent molecule AB
R = rA + rB
where R is the bond length; rAand rB are the covalent radii of atoms A and B respectively.
(ii) Bond enthalpy: The strength of a chemical bond is measured as the bond dissociation enthalpy. It is the enthalpy required to break a particular bond in one mole of a gaseous molecule. For example for a homonuclear diatomic molecule H2, we have,
Similarly for a heteronuclear diatomic molecule HCl, we have,
For molecules like O2 and N2 containing double and triple bonds:
(iii) Bond Order: According to Lewis concept of a covalent bond, the bond order is given by the number of bonds between the two atoms in a molecule. For example,
Bond order in H
2 (H – H) = 1
Bond order in O
2 (O = O) = 2)
Bond order in N
2 (N ≡ N) =3
Similarly, the bond order in CO (three shared electron pairs between C and O) is 3.
Isoelectronic molecules and ions have identical bond order; For example, F
2 and O
22–have bond order 1. N
2, CO and NO
+ have bond order 3.
It should be noted that with the increase in bond order, bond enthalpy increases and bond length decreases.
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