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Categorise the molecules which disobey the octet rule and explain them with suitable example. |
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Answer» Solution :Exception to the octet rule can be categorized into following three types. (i) Molecules with electron deficient CENTRAL atoms (ii) Moecules containing odd electrons (iii) Moleucles with expanded valence shells (i) Molecuels with electron decient centralatoms : Let us considerboron trifluoride , as an example. The central atom boron has three valence electron and each fluorine has seven valence electrons. The Lewis structure is `:underset(* *)overset(* *)(F)-underset( :underset(* *)(F: ))underset(|) B -underset(* *)overset(* *) (F):` In the above structure, only six electrons around boron atom. Moving a lone PAIR from one of the uorine to FORM additional bond as shown below. `:underset(* *)overset(* *)(F)-underset( :F: )underset(||) B -underset(* *)overset(* *) (F):` However , the above structure is unfavourable as the most electrongative atom uorine shows positive formal charge and hence the structure with incomplete octet is the favourable one. Molecules such as `BCl_(3), BeCl_(2)`, etc ... also have incomplete octets. (ii) Molecules containing odd electrons : Few molecules have a central atom with an odd number of valence electrons . For examole, in nitrogen dioxide and nitric oxide all the atoms does not have octet conguration .e lewis structure of the above molecules are shown in the figure. (iii) Moleculeswith expanded valence shells : In molecules such as sulphur hexauoride `(SF_(6))`, phosphorous pentachloride `(PCl_(5))` the central atom has more than eightvalence electrons around them. Here the central atomcan accommodate additional electronpairs by using outer vacant d orbitals. In `SF_(6)` the central atom sulphur is surrounded by six bonding pair of electrons or twelve elecrons. 
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