[nonlinear]

Hi. I understand hybridization only to the extent that I know how to count up the number of lone pairs and

What I don't understand, however, is why the nitrogen in the picture below is sp2 hybridized:


According to my textbook, it say that it is because it has an unshared pair of electrons and is adjacent to the pi bonds. Could anybody elaborate? I would have just counted up the lone pairs and the number of atoms it was bonded to, and assumed that it was sp3 hybridized.

Thank you!

[Indole]

First think about the overall shape of the molecule:

The four sp2 hybridized carbons can be visualized on a flat plane (overlap of sp2 hybridized orbitals), and each carbon has a perpendicular bonding p-orbital as well.

If you consider nitrogen, it wants to engage in three single covalent bonds in order to achieve an octet (thermodynamic stability means less energy required!). Based on your structure, this is certainly true...but nitrogen is the 5th element and therefore has 5 electrons, meaning there is a lone pair.

Spoiler: Because the rest of the molecule is "flat" (two conveniently placed pi-bonds), the nitrogen acts like it is sp2 hybridized, in the sense that it has a nonbonded lone-pair that is just itching to start flying around that pi-system.

Its a resonance situation in which the nitrogen provides pi-electrons such that Huckel's rule is fulfilled (4n+2 pi-electrons, where n=1).

If you are still asking yourself: "why doesn't it just stay sp3 hyridized?", then you're at least on the right track, and you will begin to notice these situations.

Good luck!