Chemistry 2e · Chemical Bonding and Molecular Geometry
Molecular Structure and Polarity
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In 30 seconds
A Lewis structure tells you which atoms are connected and how electrons are shared, but almost nothing about the molecule's shape. Two questions remain: What is the three-dimensional arrangement of the atoms? and Is the molecule polar or nonpolar? This topic answers both.
Molecular structure (geometry) is predicted with VSEPR theory Model in which electron groups repel and spread as far apart as possible Full entry → (Valence Shell Electron Pair Repulsion): electron groups around a central atom repel and settle as far apart as possible. Polarity is decided by bond polarity (set by Electronegativity An atom's pull on shared electrons in a bond Full entry → differences) and geometry together (whether bond dipoles cancel or add). Symmetry can cancel the dipoles (CO₂, CCl₄), or they can reinforce (H₂O, CHCl₃).
Why this matters
- Physical properties and solubility: boiling points track intermolecular forces (strongest between polar molecules), and "like dissolves like" predicts which drugs dissolve in water and which pollutants reach groundwater.
- Biology and drug action: cell membranes are built from lipids with polar heads and nonpolar tails; a drug's polarity governs how well it crosses them.
- Exam staple: "Predict the shape and polarity" is among the most common general-chemistry questions, recurring in organic chemistry and biochemistry.
The college version
Core Concepts
VSEPR: count electron domains, then spread them out
An Electron domain Any bond (of any order) or lone pair around a central atom Full entry → (electron group) is any region of high electron density around a central atom: a single bond, a double bond, a triple bond, or a lone pair each count as one domain. Domains repel and spread to maximize their separation:
| Domains | Electron-pair geometry | Bond angles |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90° and 120° |
| 6 | Octahedral | 90° |
Recipe: count domains (bonds of any order + lone pairs), look up the arrangement, then drop lone pairs to name the Molecular geometry Arrangement of atoms only, ignoring lone pairs Full entry → (the shape defined by atom positions only).
Lone pairs are invisible but not silent
Lone pairs repel more strongly than bonding pairs and compress nearby bond angles: CH₄ has four equivalent bonds at 109.5°, NH₃ (one lone pair) has H–N–H angles near 107°, and H₂O (two lone pairs) has an H–O–H angle near 104.5°. All three share tetrahedral electron-pair geometry, but their molecular geometries are tetrahedral, trigonal pyramidal, and bent.
Bond polarity comes from electronegativity differences
When two atoms with different electronegativities share electrons, the density sits closer to the more electronegative atom, creating a Bond dipole Uneven charge distribution along a bond, δ+ to δ- Full entry → — partial positive (δ+) on one end, partial negative (δ-) on the other. The quantitative measure is the dipole moment μ, charge times separation distance:
μ= Q × d
where Q is the separated charge and d the distance between charges. Dipole moments are reported in debyes (D); one debye equals 3.336 × 10-30 C · m. Identical atoms (Cl₂) give μ= 0; a very polar bond (H–F) gives a large μ.
Molecular polarity is a vector sum
Each polar bond is a vector pointing from δ+ toward δ-. The molecular dipole moment is the vector sum of all bond dipoles plus any lone-pair contribution. If the vectors cancel by symmetry, the molecule is nonpolar even when its bonds are polar:
- CO₂ is linear: the two C=O dipoles point in opposite directions and cancel (μ= 0).
- H₂O is bent: the two O–H dipoles add at an angle, so they do not cancel (μ= 1.85 D).
- CCl₄ is tetrahedral: four C–Cl dipoles cancel exactly (μ= 0).
- CHCl₃ is also tetrahedral, but three C–Cl dipoles and one C–H dipole do not cancel — it is polar (μ ≈ 1.04 D).
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Electron-pair geometry | Molecular geometry | Electron-pair geometry counts lone pairs; molecular geometry describes atoms only (H₂O: tetrahedral vs. bent) |
| Double bond = two domains | Double bond = one domain | VSEPR counts any bond order as one domain — a double bond occupies more space but counts once |
| Polar bonds guarantee a polar molecule | Polar bonds + geometry decide polarity | CO₂ and CCl₄ have polar bonds yet μ= 0 because dipoles cancel |
| Shape and polarity being separate topics | Shape feeding polarity | You must know the shape before you can sum the bond dipoles |
| Lone pairs counting as atom positions | Lone pairs affecting shape | Lone pairs compress angles but are not counted when naming molecular geometry |
| Larger electronegativity difference always = larger dipole moment | Difference sets direction, geometry sets net | Bond polarity needs Δχ; the molecular moment also depends on bond directions |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Magnets on a balloon push apart and spread evenly — that is VSEPR, and it decides a molecule's shape. Now imagine a tug-of-war: a strong atom (like oxygen) pulls the shared electrons toward itself. If all sides pull evenly, the molecule is nonpolar, like a balanced seesaw. If one side wins, the molecule gains a negative end and a positive end — polar, like a tiny battery.
Worked example
Example 1: Predict the shape of CO₂, H₂O, NH₃, and CH₄
For each molecule: draw the Lewis structure, count electron domains on the central atom, then name the electron-pair and molecular geometries.
- CO₂: central C has two double bonds = 2 domains → linear, 180°. Molecular geometry: linear.
- H₂O: central O has two single bonds + two lone pairs = 4 domains → tetrahedral electron-pair geometry; molecular geometry: bent, ~104.5°.
- NH₃: central N has three single bonds + one lone pair = 4 domains → tetrahedral electron-pair geometry; molecular geometry: trigonal pyramidal, ~107°.
- CH₄: central C has four single bonds = 4 domains → tetrahedral; molecular geometry: tetrahedral, 109.5°.
Example 2: Is HCl polar, and how much charge is separated?
HCl has an electronegativity difference of 3.16 - 2.20 = 0.96, so the bond is polar; its measured dipole moment is μ= 1.08 D and bond length is 127 pm.
Step 1 — convert μ to SI units:
μ= 1.08 D × 3.336 × 10-30 C · m1 D = 3.60 × 10-30 C · m
Step 2 — solve μ= Q × d for the separated charge Q, with d = 127 pm = 1.27 × 10-10 m:
Q = μd = 3.60 × 10-30 C · m1.27 × 10-10 m = 2.83 × 10-20 C
Step 3 — compare with the electron charge e = 1.602 × 10-19 C:
Qe = 2.83 × 10-20 C1.602 × 10-19 C ≈ 0.18
The H–Cl bond therefore behaves as though about 18% of a full electron charge were transferred from H to Cl — strongly polar covalent, not ionic. Units cancel at each step (D → C·m → C).
Key takeaways
- VSEPR: each bond (single, double, or triple) and each lone pair counts as one electron domain; 2 domains → linear (180°), 3 → trigonal planar (120°), 4 → tetrahedral (109.5°), 5 → trigonal bipyramidal, 6 → octahedral.
- Name the molecular geometry from atom positions only; lone pairs compress angles (CH₄ 109.5° → NH₃ ~107° → H₂O ~104.5°).
- Bond polarity requires an electronegativity difference; dipole moment μ= Q × d, with 1 D = 3.336 × 10-30 C · m.
- Molecular polarity = vector sum of bond dipoles; symmetric shapes (CO₂, CCl₄, BF₃) stay nonpolar despite polar bonds.
- "Like dissolves like": polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
How many electron domains does a central atom have with one double bond and two single bonds? What is the Electron-pair geometry Arrangement of all domains (bonds + lone pairs) Full entry →?
Show answer
Three domains (one double bond + two single bonds = 3) → trigonal planar.
Why is the H–O–H angle in water smaller than the H–C–H angle in methane?
Show answer
Two lone pairs on O repel more strongly than bonding pairs, compressing the angle to ~104.5°; methane has four identical bonding pairs at 109.5°.
CO₂ and SO₂ both contain polar bonds. Why is CO₂ nonpolar but SO₂ polar?
Show answer
CO₂ is linear, so its C=O dipoles point opposite and cancel. SO₂ is bent (lone pair on S), so the S–O dipoles add to a nonzero resultant — SO₂ is polar.
Convert a dipole moment of 0.44 D (the value for CO) to C·m.
Show answer
0.44 D × (3.336 × 10-30 C · m/D) = 1.5 × 10-30 C · m.
A molecule has four identical polar bonds arranged tetrahedrally. Is it polar? Explain.
Show answer
No. The four bond dipoles point toward the corners of a tetrahedron and sum to zero (exactly CCl₄).
Rank H–F, H–Cl, H–Br, and H–I from most to least polar using electronegativity.
Show answer
H–F (1.78) > H–Cl (0.96) > H–Br (0.76) > H–I (0.46).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Electron domain
- Any bond (of any order) or lone pair around a central atom
- VSEPR theory
- Model in which electron groups repel and spread as far apart as possible
- Electron-pair geometry
- Arrangement of all domains (bonds + lone pairs)
- Molecular geometry
- Arrangement of atoms only, ignoring lone pairs
- Electronegativity
- An atom's pull on shared electrons in a bond
- Bond dipole
- Uneven charge distribution along a bond, δ+ to δ-
- Dipole moment (μ)
- Charge separation measured as Q × d, in debyes
- Polar molecule
- Molecule with a nonzero net dipole moment
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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