Why does a stream of water bend?
Rub a plastic ruler on a sweater and hold it near a thin stream of water: the stream bends toward the ruler. Try the same with a stream of carbon tetrachloride (CCl₄), and almost nothing happens. Neither molecule carries a charge, so why the difference? The answer is molecular polarity.
Bond polarity
As lesson 2.3.5 showed, when two atoms have different electronegativities, the shared electrons shift toward the more electronegative atom.
- A covalent bond between identical atoms shares its electrons equally: a nonpolar bond非极性键, such as H–H or Cl–Cl.
- A covalent bond between different atoms shifts its electrons toward the more electronegative atom: a polar bond极性键, such as H–Cl or O–H. When the electronegativity difference is very small, as in C–H (0.35), the bond is only weakly polar; the rule of thumb in lesson 2.3.5 treats it as nearly nonpolar, though strictly it is still polar.
In a polar bond, the more electronegative end carries a partial negative charge, written δ−, and the other end a partial positive charge, δ+. In H–Cl, the H end is δ+ and the Cl end is δ−. A bond dipole偶极 is often drawn as an arrow pointing from δ+ to δ−, with a short crossbar at the tail that makes it look like a plus sign.
Molecular polarity
If the centers of positive and negative charge in a molecule don’t coincide, so that one end is partially positive and the other partially negative, the molecule is a polar molecule极性分子. If they coincide, it is a nonpolar molecule非极性分子.
Whether a molecule is polar depends on two things together: whether its bonds are polar, and its shape. Each bond dipole has a size and a direction, and the molecule’s dipole is their vector sum.
- Diatomic molecules: identical atoms (H₂, Cl₂) give a nonpolar molecule; different atoms (HCl, HF) give a polar one.
- CO₂: both C=O bonds are polar, but the molecule is linear, so the two bond dipoles are equal and opposite and cancel: CO₂ is nonpolar.
- H₂O: both O–H bonds are polar and the molecule is bent, so the dipoles don’t cancel; they add up to point toward the oxygen side: H₂O is polar.
- BF₃, CH₄, CCl₄: the bonds are polar, but the shapes are symmetric (trigonal planar, tetrahedral) and all the outer atoms are the same, so the dipoles cancel: these molecules are nonpolar.
- NH₃, CH₃Cl, CHCl₃: the shape isn’t symmetric (trigonal pyramidal), or the outer atoms aren’t all the same, so the dipoles don’t cancel: these molecules are polar.
A useful rule of thumb: if the central atom has no lone pairs and all the outer atoms are identical, the molecule is usually nonpolar; otherwise it is usually polar.
The size of a molecule’s polarity is measured by its dipole moment偶极矩, in units of debyes (D). Nonpolar molecules have a dipole moment of 0; HCl’s is 1.11 D and H₂O’s is 1.85 D.
Choose a molecule and watch how its bond dipoles (the short teal arrows) add up to the molecular dipole (the long red arrow). Then put it in an electric field and see what happens:
Drag the model to rotate it
Teal arrows are bond dipoles: each points from the partially positive end (δ+) to the partially negative end (δ−), with a cross at the positive end, and its length shows the electronegativity difference. The long red arrow is the dipole of the whole molecule, the vector sum of the bond dipoles.
Put these molecules in an electric field:
Why polarity matters
- Turning in an electric field: polar molecules turn in an electric field, with their partially negative ends toward the positive side. That is why the stream of water bends toward the charged ruler, while nonpolar CCl₄ doesn’t.
- Like dissolves like: polar solutes generally dissolve in polar solvents, and nonpolar solutes in nonpolar solvents. NH₃ and HCl dissolve readily in water; iodine (I₂) barely dissolves in water but dissolves easily in carbon tetrachloride; oil doesn’t mix with water.
- Intermolecular forces: the positive and negative ends of polar molecules also attract each other, which affects melting and boiling points (lesson 2.4.8).
Example 1: CO₂ versus H₂O
Both CO₂ and H₂O have polar bonds. Why is CO₂ nonpolar while H₂O is polar?
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Bond polarity. The electronegativity differences:
Both bonds are polar, with electrons pulled toward oxygen.
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CO₂’s shape. Carbon has no lone pairs, so CO₂ is linear. Its two C=O dipoles are equal and opposite and add to zero: CO₂ is nonpolar.
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H₂O’s shape. Oxygen has 2 lone pairs, so H₂O is bent (104.5°). Both O–H dipoles point diagonally toward oxygen and add up to a dipole toward the oxygen side: H₂O is polar.
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Conclusion. H₂O’s dipole moment is 1.85 D; CO₂’s is 0. Judge polarity in two steps: are the bonds polar, and does the shape let the bond dipoles cancel?
Example 2: Carbon tetrachloride versus chloroform
CCl₄ is nonpolar, but CHCl₃ (trichloromethane, also called chloroform) is polar. Why?
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Same shape. In both, carbon has no lone pairs, so both are tetrahedral.
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CCl₄: four identical bonds. The four equal C–Cl dipoles cancel in the tetrahedron: nonpolar.
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CHCl₃: one different bond. One Cl is replaced by H. The two bond polarities:
The four dipoles no longer cancel; they add up toward the three Cl atoms, so CHCl₃ is polar (1.04 D).
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Think about it. CH₂Cl₂ is often drawn flat on paper as if it were symmetric. But it is really tetrahedral, with both Cl atoms on one side and both H atoms on the other, so it is polar too (1.60 D). Don’t judge polarity from a flat drawing.
- H₂
Identical atoms: H–H is a nonpolar bond.
- HCl
Different atoms: electrons shift toward Cl, making the H end δ+ and the Cl end δ−.
- CO₂
Linear, so the two C=O dipoles cancel.
- H₂O
Bent, so the two O–H dipoles don't cancel.
- NH₃
Trigonal pyramidal: the three N–H dipoles add up toward the N side.
- CH₄
Tetrahedral with four identical C–H bonds, so the dipoles cancel.
- CHCl₃
The outer atoms aren't all the same, so the dipoles don't cancel.
- BF₃
Trigonal planar: three identical B–F bonds 120° apart cancel.
- SO₂
Sulfur has a lone pair, so SO₂ is bent and its dipoles don't cancel.
Common mistakes
Mistake 1: “A molecule with polar bonds must be polar.” CO₂, CH₄, CCl₄, and BF₃ have only polar bonds, yet all are nonpolar: their symmetric shapes make the bond dipoles cancel.
Mistake 2: “CH₂Cl₂ looks symmetric on paper, so it is nonpolar.” A structural formula is flat, but the real CH₂Cl₂ is tetrahedral, with both Cl atoms on one side, so the dipoles don’t cancel. Judge polarity from the three-dimensional shape.
Mistake 3: “Polar molecules carry a charge.” A polar molecule is neutral overall; its charge is just unevenly spread, with equal partial positive and partial negative charges at opposite ends.
Quick check
Which of these is a nonpolar molecule with polar bonds?
All four C–Cl bonds in CCl₄ are polar, but the molecule is tetrahedral, so the dipoles cancel and it is nonpolar. H₂O, NH₃, and HCl are all polar.
Iodine dissolves easily in carbon tetrachloride but barely in water. Why?
I₂ and CCl₄ are both nonpolar, while water is polar. Like dissolves like, so nonpolar iodine dissolves in nonpolar carbon tetrachloride.
Summary
- Bonds between identical atoms are nonpolar; bonds between different atoms are polar, with δ− on the more electronegative atom. A dipole arrow points from δ+ to δ−.
- A molecule’s polarity depends on both bond polarity and shape: if the bond dipoles add to zero, the molecule is nonpolar; otherwise it is polar. A central atom with no lone pairs and identical outer atoms usually gives a nonpolar molecule.
- Polar molecules turn in an electric field. Like dissolves like: polar solutes dissolve in polar solvents, and nonpolar solutes in nonpolar solvents.
On to AP (for later)
AP expects you to draw the Lewis structure, use VSEPR to find the shape, and then decide whether the molecule is polar (U2.7). Polarity in turn decides the kinds of intermolecular forces and solubility (U3.1, U3.10). A full answer mentions both that the bonds are polar and whether the shape lets their dipoles cancel; either alone is not enough.