Question

How do you determine sp sp2 and sp3 orbitals?

Answer 1

Look at the number of valance electrons.
If there are 3 valance electrons it will be `sp^2`
If there are 4 valance electrons it will be `sp^3`

Explanation:

Paired electrons typically do not form chemical bonds. (Paired electrons do form intermolecular bonds)
So the paired s electrons gain a little energy to jump into open p orbitals in order to form chemical bonds. The gain in ethalpy is balanced by stability gained by forming chemical bonds.

In Aluminium `Al_13^27` the ground state electron structure is
`1s^2 2s^2 2p^1` In the ground state Aluminum can form only 1 bond but by hybridizing the s orbital with 2 p orbital Aluminum can form three chemical bonds `(Al^+3)` . At +3 Aluminum is very stable having obtained the noble gas structure of Neon.

In Carbon `C_6^12` the ground state electron structure is
`1s^2 2s^2 2p^2` This leaves one p orbital open. In ground state Carbon can only form 2 bonds as in Carbon Monoxide. CO.
By hybridizing the 1 s and 3 p orbitals, Carbon can form 4 bonds. as in `CO_2`

So if there are only three valance electrons it is `sp^2`
If there are four valance electrons it is `sp^3`