What characteristic defines a polar molecule?

A polar molecule is defined by its net dipole moment, which arises from the unequal sharing of electrons among the atoms involved in the chemical bond. This unequal sharing results in one end of the molecule having a partial negative charge and the other end having a partial positive charge, creating a dipole. The presence of this dipole moment is what characterizes a molecule as polar.

In contrast, the other options reflect characteristics that are not aligned with the definition of a polar molecule. For example, an equal sharing of electrons leads to a nonpolar bond and consequently a nonpolar molecule, which does not exhibit a net dipole moment. Additionally, the physical state of a molecule at room temperature—such as being a gas—does not determine its polarity, and many polar substances exist in solid or liquid forms at ambient conditions. Lastly, a molecule with no regions of charge would also be nonpolar, as it lacks the necessary separation of charge that defines a polar molecule.