Which molecule is polar?

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Polarity arises when electrons are distributed unevenly in a molecule, creating partial charges that do not cancel out. This happens when bonded atoms have a difference in electronegativity, so the shared electrons are pulled toward one atom more than the other, giving that atom a partial negative charge and the other a partial positive charge. If the molecule’s shape doesn’t cancel these dipoles, there’s a net dipole moment and the molecule is polar.

The three homonuclear diatomic molecules—hydrogen gas, oxygen gas, and nitrogen gas—consist of identical atoms. Their bonds share electrons equally, so there are no partial charges and no net dipole moment. They are nonpolar.

Hydrogen fluoride has a large electronegativity difference between hydrogen and fluorine. Fluorine pulls electron density toward itself, producing a partial negative charge on fluorine and a partial positive charge on hydrogen. With only two atoms, there’s no geometry to cancel this dipole, so HF has a net dipole moment and is polar.