Chemistry: Atoms First 2e · Chemical Bonding and Molecular Geometry
Molecular Structure and Polarity
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In 30 seconds
A Lewis structure shows which atoms are connected, but not their three-dimensional arrangement. Molecular structure is that 3D geometry, predicted by VSEPR theory Valence shell electron pair repulsion; groups spread to minimize repulsion Full entry → (valence shell electron pair repulsion): electron groups around a central atom repel and spread as far apart as possible. Polarity describes how geometry distributes charge: molecules with polar bonds can still be nonpolar if their bond dipoles cancel by symmetry. This topic connects flat Lewis drawings to the 3D molecules that set boiling points, solubility, and biological activity.
Why this matters
Geometry governs chemistry. Water's bent shape makes it a polar solvent that dissolves salts; carbon dioxide's linear shape makes it nonpolar and only weakly attracted to water. Polarity determines a molecule's interactions — dipole–dipole forces, hydrogen bonding, or dispersion — which set melting and boiling points, solubility, and even how drugs cross cell membranes. Proteins and enzymes are built from the bond angles of their amino acids, and polarity often decides whether a molecule can pass through lipid bilayers. VSEPR is also the bridge to the advanced bonding theories of the next chapter.
The college version
Core Concepts
Electron groups: the foundation of VSEPR
Count the electron groups (regions of electron density) around the central atom: each single, double, or triple bond counts as one group, and each lone pair counts as one group. These groups repel each other and arrange themselves to maximize separation. The arrangement of all electron groups is the electron-pair geometry 3D arrangement of all electron groups around the central atom Full entry →; the arrangement of only the atoms is the molecular geometry 3D arrangement of only the atoms Full entry →. A lone pair occupies space but no atom sits there, which is why the two can differ.
The five base geometries
The number of electron groups fixes the electron-pair geometry and the ideal bond angles:
| Electron groups | Electron-pair geometry | Ideal angle | Example |
|---|---|---|---|
| 2 | linear | 180° | CO₂, BeCl₂ |
| 3 | trigonal planar | 120° | BF₃, H₂CO |
| 4 | tetrahedral | 109.5° | CH₄, NH₄⁺ |
| 5 | trigonal bipyramidal | 90° and 120° | PCl₅, SF₄ |
| 6 | octahedral | 90° | SF₆ |
Lone pairs change the shape
Lone pairs repel more strongly than bonding pairs (they sit closer to the nucleus), so they compress bond angles and remove "corners" from the shape. With four groups: one lone pair gives trigonal pyramidal (NH₃, about 107°); two give bent (H₂O, about 104.5°). With five groups, lone pairs prefer equatorial positions (SF₄ is see-saw, BrF₃ is T-shaped, XeF₂ is linear); with six, they sit opposite each other (XeF₄ is square planar). Subtract lone pairs from the electron-pair geometry to get the molecular geometry.
Polarity: bond dipoles and their vector sum
A bond is polar when its atoms differ in electronegativity; the bond dipole Charge separation along a polar covalent bond Full entry → points toward the more electronegative atom. The molecular dipole moment Vector sum of bond dipoles and lone-pair effects Full entry → is the vector sum of bond dipoles plus any lone-pair contribution. If dipoles cancel by symmetry, the molecule is nonpolar despite polar bonds: CO₂, BF₃, CCl₄, SF₆. If they do not cancel — H₂O, NH₃, CHCl₃ — it is polar. Lone pairs add to the dipole of NH₃ and H₂O.
How It Works / Step-by-Step Process
- Draw the Lewis structure; count electron groups around the central atom.
- Assign electron-pair geometry from the group count (2–6).
- Subtract lone pairs to find and name the molecular geometry.
- Note ideal angles, then shrink them for lone-pair repulsion.
- Find each bond dipole from electronegativity differences; add as vectors.
- Zero vector sum = nonpolar; nonzero = polar.
Common Confusions
| Do Not Confuse | With | The Difference |
|---|---|---|
| Electron-pair geometry | Molecular geometry | Electron-pair geometry includes lone pairs; molecular geometry names only the atoms |
| Polar bond | Polar molecule | A molecule with polar bonds can be nonpolar if the dipoles cancel (CO₂, CCl₄) |
| Bent (109.5° parent) | Bent (120° parent) | H₂O is bent from a tetrahedral parent; SO₂ is bent from a trigonal planar parent |
| Dipole moment | Bond polarity alone | The dipole moment is the vector sum; symmetry can cancel individual bond dipoles |
| VSEPR prediction | Actual measured angle | VSEPR gives ideal angles; lone pairs and multiple bonds compress them slightly |
| Geometry | Shape | Often used interchangeably, but electron-pair geometry and molecular geometry are distinct terms |
Eli explains
The same idea, in plain words
Explain it like I’m 10
VSEPR says electron groups around an atom behave like balloons tied together: they push each other away and spread out as far as they can, which gives molecules their shapes. A molecule is polar when one side is a bit negative and the other a bit positive, like a magnet. If the pushes cancel evenly, like two people pulling a rope with equal strength, the molecule is nonpolar even if its individual bonds are polar.
Worked example
Example 1: Predicting the shape and polarity of water (H₂O)
Oxygen has six valence electrons; with two hydrogens it forms two bonds and keeps two lone pairs — four electron groups. Four groups give tetrahedral electron-pair geometry; removing the two lone pairs leaves bent molecular geometry, with the ideal 109.5° angle compressed by lone-pair repulsion to about 104.5°. Each O–H bond is polar (oxygen is more electronegative), and the two bond dipoles point toward oxygen. Because water is bent, the dipoles do not cancel: they add to a net dipole of about 1.85 D. Water is polar — the reason it dissolves salts, hydrogen-bonds to itself, and boils high for its size.
Example 2: Why carbon dioxide is nonpolar
Carbon dioxide has two electron groups around carbon (each double bond counts as one), giving linear geometry with 180° between them. Each C=O bond is strongly polar, with the dipole pointing toward oxygen. The two dipoles are equal and opposite, so their vector sum is zero:
μ⃗total = μ⃗1 + μ⃗2 = 0
CO₂ is therefore nonpolar despite two very polar bonds — explaining its poor solubility in water and its low boiling point, since weak dispersion forces are all it has. The same logic applies to BF₃, CCl₄, and SF₆.
Example 3: Dipole moment calculation — a single bond dipole
The dipole moment of a bond is the product of separated charge and distance:
μ= Q × d
where μ is the dipole moment, Q is the magnitude of the separated charge, and d is the bond length. For hydrogen chloride, the measured dipole moment is 1.08 D and the bond length is 127 pm. Convert the bond length to meters and use the conversion 1 D = 3.336 × 10-30 C · m:
d = 127 pm × 10-12 m1 pm = 1.27 × 10-10 m
Q = μd = 1.08 D × 3.336 × 10-30 C · mD1.27 × 10-10 m = 2.84 × 10-20 C
Compare with the full electron charge (1.60 × 10-19 C): the separated charge is about 18% of a full electron, so H–Cl is roughly 18% ionic in character — most electron density is still shared. This is the standard way to quantify how "ionic" a polar covalent bond really is.
Key takeaways
- One electron group = one bond (any order) or one lone pair; count groups around the central atom.
- 2 groups → linear (180°); 3 → trigonal planar (120°); 4 → tetrahedral (109.5°); 5 → trigonal bipyramidal (90°/120°); 6 → octahedral (90°).
- Lone pairs repel more than bonds: NH₃ ≈ 107°, H₂O ≈ 104.5° (both tetrahedral electron-pair geometry).
- Molecular geometry = electron-pair geometry minus the lone pairs.
- Symmetric molecules with polar bonds can be nonpolar: CO₂, BF₃, CCl₄, SF₆.
- Polarity decides boiling point, solubility, and membrane permeability.
- Dipole moment units: debye (D); 1 D = 3.336 × 10-30 C · m.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
How many electron groups surround the central atom in CH₄, and what is its molecular geometry?
Show answer
Four electron groups; tetrahedral molecular geometry (no lone pairs).
Why is the H–O–H angle in water (104.5°) smaller than the ideal tetrahedral angle?
Show answer
Lone pairs repel more strongly than bonding pairs and compress the bond angle from 109.5° to about 104.5°.
A molecule has polar bonds but a zero dipole moment. What must be true of its geometry?
Show answer
Its bond dipoles cancel by symmetry — the molecule is symmetric (e.g., linear, trigonal planar, tetrahedral with identical groups).
What molecular geometry does a central atom with 5 electron groups and 2 lone pairs have?
Show answer
T-shaped (from trigonal bipyramidal electron-pair geometry, lone pairs in equatorial positions).
What physical property would you predict for a large nonpolar molecule compared with a similar polar one?
Show answer
Lower boiling point and poorer solubility in water, because only weak dispersion forces act between nonpolar molecules.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- electron group
- Any region of electron density: a bond of any order or a lone pair
- electron-pair geometry
- 3D arrangement of all electron groups around the central atom
- molecular geometry
- 3D arrangement of only the atoms
- VSEPR theory
- Valence shell electron pair repulsion; groups spread to minimize repulsion
- bond dipole
- Charge separation along a polar covalent bond
- dipole moment
- Vector sum of bond dipoles and lone-pair effects
- polar molecule
- Molecule with a nonzero dipole moment
Sources & references
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