What is the reason an atom hybridizes its orbitals? ?

Question

To either sp, sp2, sp3?

Is it the amount of electrons and how they fill the orbitals?

Answer 1

What obscurusvita said is true, but the most simple-minded answer is that an atom will fit into whatever "hybridization" gives the compound to be formed the lowest energy.  Hybridization is simply something that chemists invented to give us an explanation we could work with and extend in some (but not necessarily all) cases.

Answer 2

Actually an atom probably does not "hybridize" its orbitals. Hybridization is required to make the accepted spdf model which does not have tetrahedral oriented orbitals rearranged to allow even the simplest 3-D bonding to be accomplished. The electrons of atoms do move to provide the lowest energy state for the molecule. The MCAS model does not require "hybridization" as it is based on a tetrahedral base orbital instead of the spherical one currently in vogue. That dz2 orbital in the current model is a real "looker". A bit degenerate, if you check.

Answer 3

An atom will hybridize its orbitals in order to accommodate enough electrons for bonding purposes.

The hybridization is determined by the number of single bonds and lone pairs on the atom of interest.

For example, CH4 is sp3 hybridized because it has 4 single bonds...one single bond from the C to each of the H atoms

H2O is also sp3 hybridized because it has two single bonds and the oxygen has two sets of lone pairs

Ethylene CH2=CH2 ...the carbon atoms are sp2 hybridized because they only make 3 single bonds...the double bond is from the spare p orbital which overlaps with the other p orbital to form a pi bond

In acetylene HC(triple bond)CH the carbons are sp hybridized because they only make two single bonds....one to H and the other single bond to the other carbon atom....the triple bond consists of two pi bonds which are perpendicular to each other.

To summarize:

Hybridization = # of single bonds + # of lone pairs

2 = sp

3 = sp2

4 =  sp3