[thedy]

When I know a molecule has resonance structures?
I have asked soooo many times it,but I don t understand when molecule has more Lewis valid structure?And what is the condition of it?
For example I have molecule H2.Why I cannot write resonance structure for it?Because I haven t pi bond?If yes,why it is so?Why sigma electrons cannot make resonance?

have thought,that resonance structure is something like possible variations.For example in formaldehyde.There is a 3 possible: H2C=O and H2C-O::: and H2C:-O:: .So once we have double bond,and once single with three lone pairs on oxygen and the end one single bond with one lone pair on carbon and two lone pairs on oxygen.

But for example in H2,we have only one choice.Because there is only one single bond.No variations.I know it s probably stupid sight.But I m totally down with this.Please help.And Please don t give me links,because I have read,in my opinion,all web sites about this.

And peacefully be detailed and try it explain like I have poor knowledges.

On some web sites they explain it for example in this way:"Resonance is possible whenever a Lewis structure has a multiple bond and an adjacent atom with at least one lone pair",or "For some molecules and ions, it is difficult to determine which lone pairs should be moved to form double or triple bonds. This is sometimes the case when multiple atoms of the same type surround the central atom, and is especially common for polyatomic ions." or easy to say electrons are delocalised.But how strong delocalised?More then sigma electrons?Or they say:"When drawing Lewis structures, sometimes you will find that there are many ways to place double bonds and lone pairs about a given framework of atoms."---But what does "there are many ways" means?Combinations,how to bond atoms together?Or what?

Thanks

[fledarmus]

Resonance structures are just our way of admitting the limits of our bond theory.

Think about the offspring from a very small mother and a very tall father. Are they the same height as the mother or the same height as the father? They are probably neither - you would predict that they would be intermediate in height between the mother and the father. Of course, there could be genetic or nutritional factors that would push them towards either being taller or shorter.

That is what resonance structures are. If you can take a molecule and draw several reasonable resonance forms for that molecule, the actual structure of the molecule will probably be intermediate between all the forms that you can draw. The classic example is benzene, drawn as resonance structures of two cyclohexatrienes. You would expect the double bonds to be shorter than the single bonds, but if you overlay the two structures, you see that one structure has a single bond where the other structure has a double bond. The best measurements that we can make of benzene do not show two bond lengths - instead, they show that the bond length is intermediate between the two resonance structures.

So what we have done with resonance structures is to make up a short of mathematics that allows us to use all of the resonance structures we can imagine to try to predict what the actual form of the molecule would be. Single bonds, double bonds, triple bonds, +1 charges, -1 charges, these are our limitations in explaining the structures, and the true forms can be in between - a carbon-carbon bond could be mostly single bond with a little bit of double bond character and a partial negative charge, for example.

Not all resonance forms are equal - some of those forms, if the molecule existed solely in that form, would have less energy, and some would have more energy. The actual structure of the molecule will be closest to the lower energy form (they "contribute" more to the final structure), and forms which are too high in energy contribute almost nothing. If you look at keto-enol tautomers, for example, depending on the substituents your molecule may be mostly keto in character, mostly enol, or somewhere in between, just like the offspring of a tall and short parent may be fairly tall, fairly short, or in between.

[thedy]

Resonance structures are just our way of admitting the limits of our bond theory.

Think about the offspring from a very small mother and a very tall father. Are they the same height as the mother or the same height as the father? They are probably neither - you would predict that they would be intermediate in height between the mother and the father. Of course, there could be genetic or nutritional factors that would push them towards either being taller or shorter.

That is what resonance structures are. If you can take a molecule and draw several reasonable resonance forms for that molecule, the actual structure of the molecule will probably be intermediate between all the forms that you can draw.

But when I know I can draw resonance forms,and when not?When can I use princip of "small mother and tall father"?
Thanks

[fledarmus]

Eventually you develop a feel for it, but it involves looking at a lot of resonance structures and pushing a lot of electrons around.

http://en.wikipedia.org/wiki/Resonance_structures

has a number of standard examples that should be reasonably easy to understand. Look for these types of structures in other molecules that you see and start practicing drawing structures. That article gives four basic rules for identifying the good ones which are worth repeating:

Major contributors are generally structures
 

•   that obey as much as possible the octet rule (8 valence electrons around each atom rather than having deficiencies or surplus)
•   that have a maximum number of covalent bonds
•   that carry a minimum of charged atoms
•   with negative charge, if any, on the most electronegative atoms and positive charge, if any, on the most electropositive.

The greater the number of contributing structures, the more stable the molecule. This is because the more states at lower energy are available to the electrons in a particular molecule, the more stable the electrons are. Also the more volume electrons can occupy at lower energy the more stable the molecule is.

[juanrga]

But when I know I can draw resonance forms,and when not?

Exactly? When you make the quantum chemical computation.

Otherwise you will need to develop your own chemical 'intuition'

[thedy]

Eventually you develop a feel for it, but it involves looking at a lot of resonance structures and pushing a lot of electrons around.

http://en.wikipedia.org/wiki/Resonance_structures

Thanks,I m going to try it,and we will see....

[thedy]

I have forgot one question.Why resonance involves only pi electrons?Pi electrons are more moveable?As much,we can write resonance structure?
Thanks