When?

[Tatiana R Dis 3E]

How do you know when something is hybridized and how do you know which hybrid orbital it is

[Anna Goldberg 2I]

For knowing which hybridized orbital to use, consider the regions of electron density in the compound.
Linear: sp
Trigonal planar: sp2
Tetrahedral: sp3
Trigonal bipyramidal: sp3d
Octahedral: sp3d2

For compounds with lone pairs, consider the electron arrangement about the compound.
For example, a compound that is trigonal pyramidal has a tetrahedral electron arrangement (one lone pair, three bonded pairs). It thus is sp3 hybridized.

[Christine Wastila 1H]

To summarize, the number of orbitals should match the regions of electron density. So if you have AX4 or AX3E, it would be sp3 (1 s orbital + 3 p orbitals=4 total) because there are 4 regions of electron density in both cases even though one of those regions is a lone pair in the latter example.