comparing acidity of pyridinium and imidazolium cations

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I would look at it as an aromaticity question. Remember that all resonance forms are not equal, some are way bigger contributors than others. Do you think the resonance structures where the aromaticity of pyridine is broken are big contributors?

The only rule I know in this case is the positive charge on electronegative atom causes instability and here only the first structure in pyridine obeys from this, is there any more instability factors?

thanks for all the answers

I think that you may be misunderstanding or misapplying the rule that you mentioned, and you are also disregarding other rules.  A positive charge on a nitrogen atom that has a full octet is frequently observed in various Lewis structures.  All else held equal, I would rather put a positive charge onto an atom having a full octet than a positive charge onto an atom that did not.  There is also the question of aromaticity that wildfyr mentioned.  Perhaps you can start with these two ideas and explain why some of the resonance structures are minor contributors.

Quote from:

I think that you may be misunderstanding or misapplying the rule that you mentioned, and you are also disregarding other rules.  A positive charge on a nitrogen atom that has a full octet is frequently observed in various Lewis structures.  All else held equal, I would rather put a positive charge onto an atom having a full octet than a positive charge onto an atom that did not.  There is also the question of aromaticity that wildfyr mentioned.  Perhaps you can start with these two ideas and explain why some of the resonance structures are minor contributors.

So  imidazulium cation is a weeker acid because it has 2 octet forms of resonance structure but pyridine just has one octet form of resonance structure, am I right?

Yes, I believe that you are correct.

I would think in terms of the Lewis octet rule.

I would look at it as an aromaticity question. Remember that all resonance forms are not equal, some are way bigger contributors than others. Do you think the resonance structures where the aromaticity of pyridine is broken are big contributors?