Electronegativity and Bond Polarity

Electronegativity rises across a period because nuclear charge increases while electrons add to the same shell, so the nucleus pulls harder on shared electrons without much extra shielding. It falls down a group because each new period adds a shell, pushing bonding electrons farther from the nucleus and adding shielding, so the pull weakens despite the larger charge. Fluorine, top right, is the most electronegative element; francium, bottom left, is among the least. In HF, fluorine pulls the shared electron pair toward itself, leaving fluorine slightly negative and hydrogen slightly positive. That separation of charge is a bond dipole, drawn as an arrow pointing from the positive end to the negative end.

A molecule's overall polarity depends on adding up every bond dipole as a vector, not just checking whether polar bonds exist. In CO2, two equal C=O dipoles point in opposite directions across a linear molecule and cancel exactly, so CO2 is nonpolar overall even though each bond is polar. In H2O, the two O-H dipoles are equal but the molecule is bent, so they do not cancel; H2O is polar. In NH3, three N-H dipoles point down and out from nitrogen in a trigonal pyramid, and they also fail to cancel, leaving a net dipole. Symmetry is what decides cancellation, not bond polarity alone.

This sets up the rest of compound structure: once you know whether a molecule carries a net dipole, you can predict how strongly it interacts with its neighbors, which drives melting point, boiling point, and solubility in the next topics on intermolecular forces.