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109 CHEMICAL BONDING AND MOLECULAR STRUCTURE g → g g ∆ H  mol–1 2  1 –1 g → g g ∆ a 2 mol he differen e in the ∆ H alue shows a that the se ond bond undergoes some change because of changed chemical en ironment his is the reason for some differen e in energy of the same bond in different molecules like 2 5 ethanol and water herefore in olyatomi mole ules the term mean or average bond enthalpy is used It is obtained by di iding total bond dissociation enthalpy by the number of bonds bro en as e lained below in ase of water molecule, 502 + 427 Fig. 4.3 Resonance in the3O molecule erage bond enthal y 2 –1 (structures I and II represent the two canonical mol forms while the structure III is the resonance hybrid) 4.3.4 Bond Order In the Lewis description of covalent bond, In both stru tures we ha e a single the Bond Order is given by the number bond and a double bond he normal of bonds between the two atoms in a and bond lengths are m molecule. he bond order for e am le in and m res e ti ely erimentally with a single shared ele tron air in 2 2 determined o ygen o ygen bond lengths in with two shared ele tron airs and in 2 the O mole ule are same m hus the with three shared ele tron airs is o ygen o ygen bonds in the molecule are res e ti ely imilarly in three shared intermediate between a double and a single ele tron airs between and the bond bond b iously this annot be re resented order is or 2, bond order is and its by either of the two ewis stru tures shown is mol ; being one of the highest abo e for a diatomic molecule. he on e t of resonan e was introdu ed to deal with the ty e of diffi ultyerien ed Isoelectronic molecules and ions have identical bond orders; for example, F 2 and in the depiction of accurate structures of O have bond order 1. N , CO and NO have molecules like O. According to the concept bond order 3. 2 of resonance, whenever a single Lewis A general correlation useful for structure cannot describe a molecule understanding the stablities of molecules accurately, a number of structures with similar energy, positions of nuclei, bonding is that: with increase in bond order, bond enthalpy increases and bond length and non-bonding pairs of electrons are decreases. taken as the canonical structures of the hybrid which describes the molecule 4.3.5 Resonance Structures accurately. hus for , the two structures Itisoftenobser edthatasingle ewisstru ture shown abo e onstitute the anoni al is inadequate for the representation of a structures or resonance structures and mole ule in onformity with its e erimentally their hybrid i.e., the III structure represents determined arameters or e am le the the structure of O more a urately his is o one, O molecule can be equally represented also called resonance hybrid esonan e is by the structures I and II shown below: represented by a double headed arrow. Reprint 2026-27