Electronegativity and the Polar Bond
Two bonded atoms rarely pull on the shared pair equally, and the difference in pull leaves one end slightly negative. Whether the whole molecule is polar then depends on its shape, because equal pulls in opposite directions cancel.
What a learner can do afterwards
- Use a periodic trend to say which of two atoms pulls the shared pair harder
- Mark partial charges on a bond and explain what the symbols mean
- Decide whether a molecule is polar overall, given both its bond polarities and its shape
1 · Read
Picture two atoms holding one shared pair of electrons. Each atom pulls on the pair, and that pull strength is called electronegativity. Equal pulls, as in H2, leave the pair in the middle: nonpolar. A harder pull, as chlorine gives in H-Cl, drags the pair over. The puller turns slightly negative (delta minus), the other end slightly positive (delta plus). Pulls grow up and right, and fluorine pulls hardest.
The size of the pull difference works like a ruler for bond type. A zero or tiny difference means nonpolar covalent, like H-H. A middle difference means polar covalent: H-Cl differs by 0.9, and H-F differs by 1.9 yet still shares, so both are polar covalent. A very large difference hands the electron over, giving an ionic bond like Na-Cl at 2.1. Treat the ruler as rough, since some bonds sit near the lines.
Take carbon dioxide. Each bond is polar, pulling toward oxygen. But the molecule is a straight line, so the two equal pulls point in exactly opposite directions and cancel. The molecule has no overall dipole. Now take water. It is bent, so its two pulls point at an angle and add together. Water ends up with a negative side and a positive side. Same kind of bonds, different shapes, different results.
When you meet a new molecule, work in three steps. First sketch its shape: straight, bent, or something else. Then draw each bond pull as an arrow toward the stronger puller. Last, add the arrows: opposite equal arrows cancel, angled arrows combine. Try it on an N-H bond: the trend says nitrogen pulls harder than hydrogen, so nitrogen gets delta minus.
Unequal pulls make polar bonds, and shape decides if the whole molecule stays polar.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.