Hydrogen chloride (HCl) is considered a polar molecule due to the significant difference in electronegativity between the hydrogen and chlorine atoms.
In a HCl molecule, chlorine is more electronegative than hydrogen. This means that chlorine has a stronger tendency to attract electrons towards itself. As a result, the shared electrons in the H-Cl bond are pulled closer to the chlorine atom, creating a partial negative charge (δ-) on the chlorine and a partial positive charge (δ+) on the hydrogen.
This separation of charge creates a dipole moment, which is characteristic of polar molecules. The dipole moment is a vector quantity that has both direction and magnitude, pointing from the positive end (hydrogen) to the negative end (chlorine).
Therefore, because of this dipole moment and the unequal distribution of electron density, hydrogen chloride is categorized as a polar molecule. This polarity also affects its interactions with other substances, allowing it to dissolve well in polar solvents like water.