Knowing a bond is polar isn't enough to know whether the whole molecule is polar — shape matters just as much. Two polar bonds pointing in exactly opposite directions can cancel out completely, while the same two bonds at an angle will not. Think of two people pulling on a box with equal force: if they pull in opposite directions the box doesn't move, but if they both pull toward the same side, even at an angle, the box slides. Carbon dioxide and water show this perfectly. Both have two polar bonds to a central atom, yet is a nonpolar gas that barely dissolves in water, while is one of the most polar substances known. The difference is entirely shape: is straight and water is bent. To predict shape, chemists use VSEPR theory, and once you can predict shape you can predict polarity, which in turn explains boiling points, solubility and even why a microwave oven heats food.
Counting electron domains
An electron domain is any region around the central atom where electrons are concentrated. Every lone pair is one domain, and every bond to another atom is one domain, whether it is single, double or triple. That second rule surprises many students: the two double bonds in are only 2 domains, because all the electrons in a double bond sit in the same region between the same two atoms and point in the same direction. So the recipe is: draw the Lewis structure, count the atoms bonded to the central atom, add the lone pairs on the central atom, and the total sets the basic arrangement. Two domains spread to opposite sides (180°), three spread to the corners of a flat triangle (120°), and four spread to the corners of a tetrahedron (109.5°), which is the only way to get four regions equally far apart in 3-D rather than on a flat page.
| Domains | Lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | CO₂ |
| 3 | 0 | Trigonal planar | 120° | BF₃ |
| 3 | 1 | Bent | a little under 120° (about 119° in SO₂) | SO₂ |
| 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | Bent | ~104.5° | H₂O |
Lone pairs repel more strongly than bonding pairs (they're held closer to the central atom, not stretched out along a bond), which is why bond angles shrink slightly below the 'ideal' value every time a lone pair replaces a bonding pair — 109.5° → 107° → 104.5° as lone pairs are added to a 4-domain centre. Notice that lone pairs are invisible in the name of the shape. The electron-domain arrangement counts every domain, so , and all have a tetrahedral arrangement of 4 domains. The molecular shape describes only where the atoms are: with one corner of the tetrahedron occupied by a lone pair, looks like a pyramid with N at the top; with two corners occupied, is just a bent V of three atoms.
From shape to overall molecular polarity
- Draw the Lewis structure and determine the VSEPR shape.
- Identify each bond dipole (arrow pointing toward the more electronegative atom).
- Check the molecule's symmetry: if the shape is symmetric AND every outer atom is identical, the bond dipoles cancel exactly — the molecule is nonpolar overall.
- If the shape is asymmetric (including lone pairs on the central atom) OR the outer atoms are different, the dipoles do not cancel — the molecule has a net dipole and is polar.
Recognising polar and nonpolar molecules quickly
| Molecule | Domains (lone pairs) | Shape | Bond dipoles cancel? | Polarity |
|---|---|---|---|---|
| 2 (0) | Linear | Yes, equal and opposite | Nonpolar | |
| 3 (0) | Trigonal planar | Yes, three equal at 120° | Nonpolar | |
| , | 4 (0) | Tetrahedral | Yes, four identical atoms | Nonpolar |
| 4 (0) | Tetrahedral | No, one outer atom differs | Polar | |
| 4 (1) | Trigonal pyramidal | No, lone pair on one side | Polar | |
| , | 4 (2) | Bent | No, dipoles at an angle | Polar |
Why does any of this matter? Polarity controls how molecules interact with each other and with electric fields. Polar molecules line up in an electric field, which is exactly how a microwave oven works: the rapidly reversing field twists polar water molecules back and forth, and that motion becomes heat. Polarity also predicts mixing. Polar substances tend to dissolve in polar solvents and nonpolar substances in nonpolar solvents ("like dissolves like"), which is why oil, a nonpolar hydrocarbon mixture, floats on water instead of dissolving. Finally, the net dipole decides which intermolecular forces a substance has, and that sets its melting and boiling points, the topic of the next lesson.