Hybridization — sp, sp2, sp3, sp3d & sp3d2

In introductory chemistry, hybridization is the bridge from a Lewis structure to the local geometry around one atom. You count electron domains around that atom and match the count to a label:

A single bond, double bond, triple bond, or lone pair each counts as one domain. The label names how many orbitals are mixed in the model: $sp$ is one $s$ and one $p$, $sp^2$ is one $s$ and two $p$, $sp^3$ is one $s$ and three $p$, while $sp^3d$ and $sp^3d^2$ add one and two $d$ orbitals in the usual general-chemistry picture.

Why the domain count works

The count works because hybridization and VSEPR rest on the same idea: electron domains repel and arrange themselves to stay as far apart as possible. A given number of domains forces a particular geometry, and that geometry corresponds to mixing a particular number of orbitals. So counting domains, assigning the label, and predicting the shape are three readings of one underlying arrangement. This also explains why a multiple bond counts as a single domain: the two or three shared pairs occupy one region of space pointing in one direction, so they push other domains as a unit.

Worked example: carbon in $C_2H_4$ is $sp^2$

In ethene, $C_2H_4$, each carbon bonds to two hydrogens and to the other carbon through a double bond. Count the domains around one carbon:

• two $C-H$ single bonds $\rightarrow$ 2 domains
• one $C=C$ double bond $\rightarrow$ 1 domain

That is 3 electron domains, so the carbon is $sp^2$, with a trigonal planar arrangement. The example highlights the rule students miss most: the double bond counts as one domain, not two.

Try a few yourself

Run the same count on three molecules and check each against the table:

• $CO_2$: central carbon has two $C=O$ double bonds, $2$ domains $\to sp$, linear.
• $CH_4$: central carbon has four $C-H$ single bonds, $4$ domains $\to sp^3$, tetrahedral.
• $SF_6$: central sulfur has six $S-F$ single bonds, $6$ domains $\to sp^3d^2$, octahedral.

In each case the VSEPR geometry should tell the same story as the hybridization label.

Calculation pitfalls

• Counting a multiple bond as more than one domain. One double bond is one domain; one triple bond is one domain.
• Forgetting lone pairs. An atom with three bonds and one lone pair has 4 domains, often giving $sp^3$.
• Mixing up local geometry and whole-molecule shape. Hybridization is assigned one atom at a time and describes the local arrangement, not the overall shape.
• Treating every label as equally reliable in advanced cases. Labels like $sp^3d$ and $sp^3d^2$ are course-level shortcuts for 5 and 6 domains; some hypervalent molecules are described with other bonding models in later courses.

Hybridization helps most when connecting a Lewis structure to local geometry, expected bond angles, or the idea that a double bond contains one $\sigma$ and one $\pi$ bond. It helps less when bonding is strongly delocalized or a localized Lewis structure is already a weak description.