Hybridization is the model that explains why methane’s bond angles are 109.5° instead of 90°. Pure 2p orbitals would give carbon perpendicular bonds; we observe a tetrahedron. The fix Pauling proposed in the 1930s was that carbon mixes its 2s and three 2p orbitals into four equivalent sp³ hybrids pointing at the corners of a tetrahedron. The model isn’t pretty quantum mechanics — it’s a useful fiction that produces correct geometries and bond angles. Once you can count electron domains, you can predict the hybridization of any central atom in seconds.

The lookup table

Domains	Hybridization	Electron geometry	Bond angle	Example
2	sp	linear	180°	BeCl₂, CO₂
3	sp²	trigonal planar	120°	BF₃, C₂H₄
4	sp³	tetrahedral	109.5°	CH₄, NH₃, H₂O
5	sp³d	trigonal bipyramidal	90° / 120°	PCl₅, SF₄
6	sp³d²	octahedral	90°	SF₆, XeF₄

Five domains and beyond require d-orbital participation, which is only available to period 3 and beyond — that’s why sulfur can be SF₆ but oxygen can never be OF₆.

The recipe

Draw the Lewis structure with every bond and every lone pair on the central atom.
Count electron domains: each bond is one domain regardless of whether it’s single, double, or triple. Each lone pair is one domain.
Look up the hybridization from the count.
(Optional) Note the molecular geometry, which differs from the electron geometry whenever lone pairs are present.

Worked example: water

H₂O. Oxygen has 6 valence electrons → 2 in O–H bonds, 4 as two lone pairs.

Domains on O: 2 bonds + 2 lone pairs = 4 → sp³.

Electron geometry tetrahedral; molecular geometry bent (you don’t “see” lone pairs). Bond angle ≈ 104.5° — slightly squeezed below 109.5° because lone pairs repel more strongly than bond pairs.

Worked example: carbon dioxide

O=C=O. Two double bonds, no lone pairs on C.

Domains on C: 2 → sp. Linear, 180°. The two unhybridized 2p orbitals on C form the two π bonds with the oxygens.

Worked example: formaldehyde

H₂C=O. Two C–H singles + one C=O double, no lone pairs on C.

Domains on C: 3 → sp². Trigonal planar, ~120°. The leftover 2p forms the π bond with O.

Worked example: ammonia

NH₃. Three N–H + one lone pair on N.

Domains: 4 → sp³. Electron geometry tetrahedral; molecular geometry trigonal pyramidal (one corner is a lone pair). Bond angle ≈ 107° — the lone pair compresses the H–N–H angles below tetrahedral.

Worked example: phosphorus pentachloride

PCl₅. Five P–Cl bonds, no lone pairs on P.

Domains: 5 → sp³d. Trigonal bipyramidal: three equatorial Cl at 120°, two axial Cl at 90° to the equatorial plane. Phosphorus can do this because it has empty 3d orbitals; nitrogen (no 2d) cannot — NCl₅ doesn’t exist.

Worked example: xenon difluoride

XeF₂. Two Xe–F bonds + three lone pairs on Xe.

Domains: 5 → sp³d. Electron geometry trigonal bipyramidal — but the three lone pairs all sit in the equatorial plane (lone pairs prefer equatorial because they have more room there). The two F atoms occupy the axial positions.

Result: a linear molecule (180°) despite sp³d hybridization. This is the classic illustration that hybridization predicts electron geometry, not molecular geometry. You always need the lone pair count to get the actual shape.

Organic shortcut

For organic molecules, count by the number of atoms bonded to the carbon:

4 atoms attached, all single bonds → sp³ (alkanes, alcohols, amines)
3 atoms attached, including one double bond → sp² (alkenes, carbonyls, aromatic carbons)
2 atoms attached, with a triple bond or two double bonds → sp (alkynes, allenes, CO₂’s carbon)

In acetic acid (CH₃COOH):

The methyl carbon: 4 single bonds → sp³
The carboxyl carbon: 3 attached atoms (=O, –O, –C) with one double bond → sp²

In one molecule, two carbons with different hybridizations. That’s normal.

Traps people fall into

Counting double or triple bonds as multiple domains. A C=O is one domain, not two. A C≡N is one domain, not three. The rule is “regions of electron density radiating from the central atom” — π bonds piggyback on the σ direction.
Forgetting lone pairs. NH₃ is sp³ (4 domains), not sp² (3 bonds). The lone pair on N is a domain that demands a hybrid orbital just like the bonds do.
Confusing electron geometry with molecular geometry. Hybridization comes from total domains. Molecular shape comes from where the atoms sit, ignoring lone pairs. Water is sp³ (tetrahedral electron geometry) but bent (molecular geometry). Both descriptions are correct; they answer different questions.
Assigning sp³d or sp³d² to second-row atoms. N, O, F have no accessible d orbitals. They cap at four domains.

Practice

Try these against the Electron Configuration Calculator:

Hybridization of B in BF₃?
Hybridization of each carbon in CH₃CHO (acetaldehyde)?
Hybridization and molecular geometry of S in SF₄?
Hybridization of N in NO₃⁻?
Hybridization of I in ICl₄⁻ — and how many lone pairs does it have?

How to Determine Hybridization