CO2 has two polar bonds and is a nonpolar molecule. Water has two polar bonds and is one of the most polar molecules there is. Same ingredient, opposite result — the difference is shape. By the end of this page you'll be able to take any molecule's bonds and shape and predict whether it's polar, and which intermolecular force that unlocks.
Here's a trap worth naming before you fall into it: the bonds are polar, so the molecule is polar. Sometimes. Electronegativity difference (ΔEN) tells you whether a single bond dipole exists — one tug-of-war, judged bond by bond. Molecular polarity asks a different question: do all of a molecule's tugs-of-war add up to a net pull across the whole thing? Pulls in opposite directions cancel. Pulls in the same general direction don't.
CO2 is the clean case. Each C=O bond really is polar — oxygen wins the tug-of-war, so oxygen ends up δ− and carbon δ+. But CO2 is a straight line, and the two oxygens sit on exactly opposite sides of carbon. Their pulls point in exactly opposite directions and cancel — not approximately, exactly. Nonpolar molecule, polar bonds and all.
Water refuses to line up the same way. Its two O–H bonds sit at about 104° to each other — bent, not straight. Both pulls point generally toward oxygen, so instead of canceling they gang up into a net dipole. The oxygen end of the molecule runs δ−, the hydrogen end runs δ+. Water is polar because of its shape — not because O–H (ΔEN 1.24) is somehow "more polar" than C=O (ΔEN 0.89). Both bonds are genuinely polar. The difference is entirely geometric.
Molecular polarity takes two inputs: how polar the bonds are (ΔEN), and where they point (shape). Polar bonds in a symmetric arrangement usually cancel — nonpolar molecule. Polar bonds in a lopsided arrangement don't — polar molecule. Skip either input and you'll get CO2 wrong, or water wrong, or both.
Same two questions every time: is the bond polar, and does the shape let those pulls cancel?
Pick a molecule. Judge its bond, then judge what its shape does to that bond's pull. The verdict only appears after you've committed to both.
Test your understanding of bond polarity and shape. Sort each molecule into the correct category below.
Stuck on one? Tap Reveal. The point is pulling it from your head, not recognizing it on the page.
Electronegativity difference (ΔEN) tells you whether a single bond has a . Molecular shape decides whether those bond dipoles cancel or . A perfectly symmetric arrangement of polar bonds makes their pulls , giving a nonpolar molecule. A polar molecule picks up forces on top of the London dispersion forces every molecule has. Hydrogen bonding only switches on when hydrogen sits directly on nitrogen, oxygen, or .
Shape → molecular polarity → IMF type → energy needed to separate molecules → physical state. The common shortcut — "water has O–H bonds, so it has hydrogen bonds" — gets the right answer for the wrong reason, and it'll fail you on the next molecule. Write out every link before you check.
Give yourself a point for each idea you actually wrote down. The flag (⚑) marks the move that separates a full-credit answer from a partial one.
Methane (CH4) boils at −162°C. Ammonia (NH3) boils at −33°C — about 130 degrees higher — even though the two molecules have nearly identical mass (16.05 vs. 17.04 g/mol). Construct the full argument for the difference: bonds → shape → molecular polarity → IMF → boiling point.
Self-score: 4 = all four · 3 = missing the shape→polarity link · 2 = bonds + IMF, no shape reasoning · ≤1 = boiling points restated only.
Microwave ovens operate on molecular polarity. Microwaves cook food by emitting radiation that matches the natural rotational frequency of polar water molecules. The waves make the water's net dipoles spin rapidly, generating heat through friction, while nonpolar containers (like plastics) are left cold and unaffected.