Re: what is the need of resonance structure to explain prperties of molecules?

Resonance structures are not needed to describe the structure and 
properties of all molecules or ions.  However, in those cases where they 
are used, it is true that no resonance form is a complete picture or 
explanation of the structure and properties of molecules.  However in 
those same cases, an understanding of the hybrid picture formed from the 
contribution of the various resonance forms in a collection for a 
substance can be used in helping to describe the known features of the 
structure and properties of a particular molecule or ion.

Think of each of the structures in a collection of resonance forms for a 
molecule or ion as an individual structure in that collection that can 
contribute to some degree to the explanation of the structure and 
properties of that molecule or ion.  No individual structure is complete 
and therefore true, but the hybrid structure formed from the collection 
can approach closer to what we think of as the true structure.

I will give two analogies.  In the first consider a mule, the animal that 
arises from the mating of a donkey and a horse.  We can describe the mule 
by citing certain characteristics of the donkey, for example the long 
ears, and certain characteristics of the horse, such as the size of a 
horse.  By using these characteristics, we can come up with a picture of 
what a mule might look like, although neither the picture of the donkey 
nor the picture of the horse would be a true picture of a mule.  We say 
that the mule is a hybrid of a donkey and a horse.  We actually refer to 
the true structure as a "hybrid" of the collection of the resonance 
structures.

The second analogy is a bit more far fetched, but still useful in its own 
right.  Imagine that you are trying to describe to a friend what a 
rhinoceros is.  You might say that it has a horn on top of its nose like 
a unicorn has a horn on its nose, and the rhinoceros has a body like that 
of one of the dinosaurs (triceratops?).  The unicorn is a mythical animal 
that never existed and the dinosaur is one that does not exist now, yet 
each might be useful in describing the characteristics of a rhinoceros, 
which actually exists now. This analogy is not perfect, because a 
rhinoceros is certainly not a hybrid of a unicorn and a dinosaur.

There is much more to the whole concept of resonance structures, but I 
have tried to restrict my answer to the question asked.