[Billqaz3]

Given a large organic molecule with a nitrogen bonded to a carbon (all other parts of the molecule are irrelevant). The nitrogen has 3 bonds and 1 lone pair while the Carbon has 3 bonds. You shift the lone pair from the nitrogen to form a double bond with carbon to create a resonance structure.

This is what my book illustrates, my problem with this is that it also states that all resonance structures have to have the same bond angles. Wouldn't shifting the lone pair cause the nitrogen to change from a tetrahedral to a trigonal planer, thus causing a change in bond angle. So is changing angles allowed or is this a special case?

[spirochete]

Changing bond angles is never allowed. Two resonance structures are the exact same molecule, drawn in a different way. It is one molecule, at one point in time, so the shape cannot change.

For a lone pair to be part of a conjugated system (part of resonance) it needs to be in a p orbital. So a tetrahedral (sp3) atom can't be part of the conjugated system. If an atom "looks" sp3 in one structure, and sp2 in another, you should go with sp2. The atom is generally 100% sp2 hybridized and trigonal, or close to trigonal.

[Billqaz3]

Wait I don't understand I thought when calculating bond angles the loan pair counted as a group so when you have 3 sigma bonds and 1 lone pair the shape is considered tetrahedral and thus the bond angle is around 109 degrees. But when that lone pair forms a double bond the number of groups decreases to 3 causing the bonds to be 120 degrees

[Corribus]

First, a nitrogen in a conjugated heterocycle (like pyridine) is not tetrahedral. The lone pair on the nitrogen is in the molecular plane, in an sp2 hybridized orbital.

The structure doesn't change as you draw alternative resonance structures because resonance structures do not represent discrete electronic states. They are alternating ways to draw electron locations* that are energetically equivalent. The true electronic state is a time-average of the possible resonance structures. (Another way to view it is that electron motions are far quicker than nuclear motions, electrons being far smaller, and therefore the molecular skeleton does not have time to change position in response to electron trajectories (as far as trajectory has any meaning for electrons... which it doesn't...)).

Do also realize that resonance, like all bonding theories, is a conceptual model built to help us understand molecular structure - and, for organic chemists, to help predict and understand reaction behavior of complex organic systems. A molecular orbital approach, something very different, has very little use for the resonance concept - but after you've been exposed to a number of bonding theories, you will realize that most of them use different language to arrive at more or less the same (qualitative) conclusion. This is a good thing, of course. If different theories led to wildly conflicting conclusions, then we'd have problems. This has little bearing on your question, of course, but I think it's always good to keep in mind.

*And really, it's mostly a method of bookkeeping.

[spirochete]

Wait I don't understand I thought when calculating bond angles the loan pair counted as a group so when you have 3 sigma bonds and 1 lone pair the shape is considered tetrahedral and thus the bond angle is around 109 degrees. But when that lone pair forms a double bond the number of groups decreases to 3 causing the bonds to be 120 degrees

You're describing an over simplified approach to predicting bond angles for molecules. This is perfectly fine for many molecules, and it's a great starting point. AFAIK many general chemistry classes will simply end there, along with a few extra notes about lone pair repulsion vs. bond pair repulsion to estimate how the HOH bond angle in water is less than 109.5 degrees.

Like I said above, when a lone pair (or any electron pair) is delocalized over several atoms, it needs to be in a p orbital. An atom that's part of a conjugated system cannot be sp3 hybridized. Consider the description of a formate ion here: http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_02%3A_Introduction_to_organic_structure_and_bonding_II/Section_3%3A_Resonance

And as corribus said, talking about the hybridization of a particular atom is also an over simplification. Molecular orbitals are more realistic probably, but that's even more confusing.