VSEPR feels abstract until it's connected to a simple physical idea: electron groups around an atom repel each other and spread out as far apart as possible. Everything else in this topic follows from that one idea.
Counting electron domains
Start from a correct Lewis structure. Count every group of electrons around the central atom as one domain, whether it's a single bond, a double bond, a triple bond, or a lone pair. A double or triple bond still counts as only one domain, since all the electron density from that bond occupies the same general region.
From domain count to shape
| Domains | Electron domain geometry | With one lone pair |
|---|---|---|
| 2 | Linear | — |
| 3 | Trigonal planar | Bent |
| 4 | Tetrahedral | Trigonal pyramidal |
Electron domain geometry counts every domain, lone pairs included. Molecular geometry only names the arrangement of the actual atoms, so a central atom with lone pairs has a different molecular geometry name than its electron domain geometry, even though the underlying electron arrangement is identical.
Hybridization follows directly from domain count
Two domains correspond to sp hybridization, three to sp2, four to sp3, five to sp3d, and six to sp3d2. Since hybridization is really just a description of how atomic orbitals combine to make room for however many electron domains are present, knowing the domain count answers both the shape question and the hybridization question at once.
Where students actually lose points here
Counting a double or triple bond as more than one domain
A double bond is still one region of electron density for VSEPR purposes, even though it involves two shared pairs of electrons. Counting it as two domains gives the wrong shape entirely.
Forgetting lone pairs still affect bond angles
Lone pairs aren't named in the molecular geometry, but they still take up space and compress the angles between the actual bonds, since lone pair repulsion is slightly stronger than bonding pair repulsion.
Mixing up electron domain geometry and molecular geometry
These are genuinely different answers to different questions, and a question asking specifically for "molecular geometry" wants the atoms-only shape, not the full electron domain arrangement.
One idea that unlocks three questions at once
Once electron domain counting is solid, shape, bond angle, and hybridization questions all become the same quick lookup instead of three separate things to memorize.
Common questions
Does a double bond count as one or two electron domains?
One. A double or triple bond still occupies a single region of electron density for VSEPR purposes, even though it involves more than one shared pair of electrons.
What's the difference between electron domain geometry and molecular geometry?
Electron domain geometry counts every domain around the central atom, including lone pairs. Molecular geometry only describes the arrangement of the actual bonded atoms, so a molecule with lone pairs gets a different molecular geometry name even though the underlying domain arrangement is the same.
How do you find hybridization from a Lewis structure?
Count the electron domains around the central atom. Two domains means sp, three means sp2, four means sp3, five means sp3d, and six means sp3d2.