Hybridized Orbital Theory was developed to correctly account for the geometry of molecules and the bond angles. According to this theory, when an atom forms bonds in a molecule, its atomic orbitals mix together to form hybrid orbitals. Examples of hybrid orbitals are sp, sp2, and sp3.
An sp orbital is formed from the mix of an s and one p orbital.
Sp2 is formed when one s and two p orbitals mix ( px, py)
Sp3 is formed when one s and three p orbitals mix (px, py, and pz)
To correctly determine hybridization, we must fist figure out the steric number.
Steric Number = Number of Attachments + Number of Electron Lone Pairs
When steric number is 2, hybridization is sp
When steric number is 3, hybridization is sp2
When steric number is 4, hybridization is sp3
Exception: When an atom that appears to be sp3 hybridized has a lone pair adjacent to a pi bond, that atom is in fact sp2 hybridized.
Steric Number = number of attachments + number of lone pairs