Lesson 5 of 7 · Molecular structure
Lewis structures & polarity
A Lewis structure shows how valence electrons form bonds and lone pairs. VSEPR (valence-shell electron-pair repulsion) theory helps predict the molecule’s shape. To decide whether a molecule is polar, consider both its bond polarities and how those bonds are arranged in space.
Shape changes polarity
Compare water and carbon dioxide in the schematic vector diagram, or launch the interactive 3D stage to inspect H₂O, CO₂, and CH₄ from any angle.
In these teaching models, oxygen is crimson-pink, carbon is slate and hydrogen is white. Atom sizes and bond lengths are illustrative. Drag the 3D model to orbit or use the rotation buttons.
Build the Lewis structure
Count the valence electrons from every atom, then add electrons for a negative charge or subtract them for a positive charge. Choose a central atom, usually the least electronegative atom other than hydrogen, and connect the surrounding atoms with single bonds. Complete the outer atoms’ shells where possible, then place any remaining electrons on the central atom. Hydrogen needs two electrons; most second-period atoms are drawn with eight.
If the central atom still lacks an octet, a neighboring lone pair may form a double or triple bond. Check that the drawing uses the right number of electrons. The octet rule has exceptions, so do not force eight electrons onto every atom.
Check the electron bookkeeping
Formal charge is a way to check a Lewis structure: take the valence-electron count of the free atom, subtract its nonbonding electrons, then subtract half its bonding electrons. The formal charges must add up to the overall charge of the molecule or ion.
Some molecules and ions need more than one Lewis drawing to describe their bonding. These are resonance structures: the atoms stay in place, but the drawn electron positions change. The real electrons are spread over the structure; the molecule does not switch back and forth between the drawings.
Reference: OpenStax Chemistry 2e, §7.4: Formal charges and resonance
Count electron domains
An electron domain is a region of electron density around the central atom. Each bond counts as one domain, whether it is single, double or triple; each lone pair also counts as one. Electron-domain geometry includes all these regions. Molecular geometry names the arrangement of the atoms only.
| Molecule | Central domains | Molecular shape | Bond angle |
|---|---|---|---|
| CO₂ | 2 bonds, 0 lone pairs | Linear | 180° |
| CH₄ | 4 bonds, 0 lone pairs | Tetrahedral | 109.5° |
| NH₃ | 3 bonds, 1 lone pair | Trigonal pyramidal | ≈107° |
| H₂O | 2 bonds, 2 lone pairs | Bent | ≈104.5° |
Reference: OpenStax Chemistry 2e, §7.6: Structure and polarity
Add the bond dipoles
A bond dipole describes the separation of positive and negative charge along a bond. Picture it as a small arrow pointing toward the more electronegative atom. In linear CO₂, the two equal bond dipoles point in opposite directions and cancel. In bent H₂O, they do not cancel, so water has a net dipole and is polar.
The 3D view above uses a ball-and-stick model to make the geometry easier to see. Atom colors and sizes are illustrative, and the angles are rounded classroom values. Use it to compare the displayed molecules; it does not calculate the shape or energy of a new molecule.
Reference: OpenStax Chemistry 2e, §7.6: Structure and polarity
Work through the examples
Worked example 1
Why is water bent?
- Count 8 valence electrons: 6 from O and one from each H.
- Two O–H bonds use four electrons; four remain as two lone pairs on O.
- Four electron domains give a tetrahedral domain arrangement, but only two domains contain bonded atoms.
Answer: H₂O has a bent molecular shape and an H–O–H angle of about 104.5°.
Worked example 2
Why is carbon dioxide nonpolar?
- The Lewis structure O=C=O gives two bonding domains at carbon.
- Two domains give a linear geometry.
- Equal bond dipoles point in opposite directions.
Answer: The net dipole moment is zero for the isolated linear CO₂ molecule.
Avoid these mistakes
- Do not count a double bond as two VSEPR domains.
- Do not use “tetrahedral” for water’s molecular shape; that describes its electron-domain geometry.
- VSEPR is a qualitative model, especially limited for many transition-metal complexes.
Check your understanding
Open printable worksheetTry each question before opening the answer key. When you check your answers, read the explanations too. Your choices stay in this browser.
Read the complete answer key
1. A — 1. A multiple bond is one region of electron density for domain counting.
2. C — Trigonal pyramidal. Three bonding domains and one lone pair give a trigonal pyramidal molecular shape.
3. C — H₂O. Water is bent, so the O–H bond dipoles do not cancel. Symmetry cancels the dipoles in CO₂ and CH₄.
4. C — 2. After the two O–H bonds, four of the eight valence electrons remain as two lone pairs on oxygen.
Common questions
Is methane polar?
An isolated CH₄ molecule has a symmetric tetrahedral shape, so its bond dipoles cancel and it has no permanent net dipole moment.
Are the model angles exact for all conditions?
No. Ideal geometries and rounded classroom values are useful approximations. Measured geometry depends on the species and physical conditions.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.