Lewis Structures and Molecular Geometry
Count valence electrons, build a defensible Lewis structure, count central-atom domains, and predict shape and polarity.
Lewis Structures and Molecular Geometry
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Lewis structure first; electron domains second; molecular shape third
Step 1: Count all valence electrons. Step 2: Choose a central atom—usually the least electronegative atom that is not hydrogen. Step 3: Connect atoms, complete terminal octets, and place remaining electrons on the center. Step 4: Form multiple bonds when needed to improve octets and formal charges.
For VSEPR, every single, double, or triple bond counts as one electron domain; every lone pair also counts as one. Electron geometry counts all domains, while molecular geometry names the positions of atoms only.
Formal charge checks the structure
Formal charge = valence electrons − nonbonding electrons − number of bonds. Prefer structures with small formal charges and negative charge on the more electronegative atom when possible.
Shape controls dipole cancellation
Polar bonds do not guarantee a polar molecule. Symmetric arrangements such as CO₂, BF₃, and CH₄ can cancel equal bond dipoles.
Use the domain code around the central atom
2 domains: Linear, 180°.
3 domains: Trigonal planar electron arrangement, about 120°.
4 domains: Tetrahedral electron arrangement, about 109.5°.
Then hide the lone pairs when naming the molecular shape: four domains give CH₄ tetrahedral, NH₃ trigonal pyramidal, and H₂O bent.
Why it works
The domain count separates the underlying electron arrangement from the visible molecular shape and prevents students from treating every four-domain molecule as tetrahedral.
Five forms you should recognize
Carbon has two double bonds. Each double bond counts as one domain, giving two domains and a 180° linear molecule. Equal C=O dipoles cancel.
Boron has three B–F bonding domains and no lone pairs. The bonds spread about 120° apart in one plane.
Four C–H bonding domains and no lone pairs point toward the corners of a tetrahedron, about 109.5° apart.
Nitrogen has three bonds and one lone pair. Four total domains give tetrahedral electron geometry, but the atoms form a trigonal pyramid.
Oxygen has two bonds and two lone pairs. Four total domains give tetrahedral electron geometry, while the atoms form a bent shape near 104.5°.
Check before you commit
- Counting a double bond as two electron domains
- Forgetting hydrogen follows a duet rather than an octet
- Naming electron geometry when the question asks for molecular shape
- Ignoring lone pairs on the central atom
- Assuming every molecule with polar bonds is polar overall
- Changing the total valence-electron count to make a drawing convenient
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Lewis Structures and Molecular Geometry FAQ
Why is a double bond only one VSEPR domain?
Its electron density occupies one general direction from the central atom, even though it contains two shared pairs.
Why is NH₃ not trigonal planar?
Its fourth electron domain is a lone pair, which pushes the three N–H bonds into a trigonal-pyramidal shape.
Can a molecule have polar bonds but be nonpolar?
Yes. Symmetry can make equal bond-dipole vectors cancel, as in CO₂ or BF₃.
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