polarizability
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Hold a charged balloon near a thin stream of tap water and the stream bends toward it, even though water carries no net charge. The balloon's field nudges the electrons in each molecule, squeezing them slightly to one side. How easily a molecule's electron cloud can be pushed and squashed like this is its polarizability — a measure of how 'soft' and squishy its electrons are.
Polarizability is the ease with which an external electric field distorts a molecule's electron cloud, inducing a temporary dipole where there was none. A big, loosely held electron cloud — many electrons, far from the nucleus, weakly bound — is highly polarizable; a small, tightly held one is not. Larger atoms and molecules, and those lower in the periodic table, are generally more polarizable.
Why it matters: polarizability is the source of London dispersion forces, so it largely sets the boiling points of nonpolar substances and explains why heavier, bigger molecules stick together more. It also shows up in how light bends through a material (refractive index) and in spectroscopy. The honest caveat is to keep it separate from a permanent dipole moment: polarizability is about how easily a dipole can be induced, not about a dipole that is already there.
Iodine molecules are far more polarizable than fluorine molecules because iodine's many outer electrons are loosely held, which is why iodine is a sticky solid while fluorine is a gas.
Bigger, looser electron clouds are more polarizable and stickier.
Polarizability (induced dipole) and dipole moment (permanent dipole) describe different things. A nonpolar molecule with zero dipole moment can still be highly polarizable — that is exactly how dispersion forces work in oils, fats, and noble gases.