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I do not understand why and how an atom with $\ce{sp^2}$ hybridization has only one p orbital??

For example in pyridine why is the lone pair of nitrogen not counted as a pi electron.

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The picture below from the UC Davis ChemWiki summarizes why that lone pair is not available. It also explains how pyridine can function as a base. pyridine orbitals

The lone pair in the $sp^2$ orbital is perpendicular to the $p$ orbitals and $\pi$ bonds in the molecule. the electron pair from the nitrogen that participates in aromaticity is the one in the $\ce{C=N}$ bond.

To answer your first question about hybridization, we have a conservation law that guarantees that when orbitals mix (for any reason), we need to get the same number of orbitals out as we put in. The law is the Conservation of Angular Momentum, which does apply in quantum systems.

Nitrogen, in its valence shell, has four orbitals: $2s,2p_x,2p_y,2p_z$. The $sp^2$ hybridization is achieved by mixing two $p$ orbitals and one $s$ orbital. Because of the conversation of angular momentum, four orbitals in means four orbitals out. Thus, the $p$ orbital that was not mixed is still there (and is the only one still there).

$$[s,p,p],p\implies[sp^2,sp^,sp^2],p$$

We cannot have two $p$ orbitals at an $sp^2$ hybridized atom because then we would have 5 orbitals (or we would not have $sp^2$).

Now, let's create a general rule for aromatic heterocycles.

Look at pyrrole (also from ChemWiki);

pyrrole

In the structure of pyrrole, the $\ce{N-H}$ bond uses the $sp^2$ orbital and the lone pair is in the $p$ orbital. In general, if there is a double bond drawn to the nitrogen atom (like in pyridine), then the $\pi$ bond uses the $p$ orbital and the lone pair is in the $sp^2$ orbital and does not count toward aromaticity. If there is no double bond drawn to the nitrogen atom, usually there is a $\ce{N-H}$ bond (or some other bond to the nitrogen). This $\ce{N-H}$ bond uses the $sp^2$ orbital and the lone pair uses the $p$ orbital and counts toward aromaticity.

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  • $\begingroup$ Good answer.But I have a doubt.If an acid is added to pyridine,will the aromaticity be preserved or destroyed as in case of pyrrole ? $\endgroup$ – user14857 Dec 22 '15 at 5:19
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    $\begingroup$ Pyridine remains aromatic when protonated. The lone pair that enables pyridine to be a base is orthogonal to the pi system. $\endgroup$ – Ben Norris Dec 22 '15 at 11:19
  • $\begingroup$ Actually pyrrole gets C-protonated IIRC $\endgroup$ – Mithoron Jul 13 '17 at 15:16
  • $\begingroup$ @Mithoron - it likely does, but either way, aromaticity is lost. $\endgroup$ – Ben Norris Jul 13 '17 at 22:37

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