Molecular polarity determines physical properties like solubility and boiling point. This property arises from the distribution of electrical charge across the molecule’s structure. Molecular symmetry and charge distribution provide the necessary framework to classify any molecule, such as H₂, as either polar or nonpolar.
The Core Concept: Molecular Polarity
A molecule is polar if it possesses an uneven distribution of electrical charge, creating a positive and a negative end. This separation is quantified as an electric dipole moment. In contrast, a nonpolar molecule has its electrical charge distributed uniformly and symmetrically, resulting in a net zero dipole moment.
Molecular polarity depends on the combined effect of individual bond polarity and the overall three-dimensional shape. Even if a molecule contains polar bonds, the molecule can be nonpolar if its geometry is symmetrical, causing the bond dipoles to cancel out. For instance, carbon dioxide (CO₂) has two polar bonds, but its linear shape makes the molecule nonpolar overall because the charge separation vectors pull equally in opposite directions.
Electronegativity and Bond Types
Bond polarity is created by electronegativity, which is an atom’s measure of its ability to attract a shared pair of electrons within a chemical bond. When two atoms with different electronegativity values bond, the shared electrons are pulled closer to the atom with the higher value, creating a separation of charge.
The difference in electronegativity determines the type of bond formed. A large difference (generally above 1.7) results in an ionic bond where electrons are transferred. A moderate difference (typically between 0.4 and 1.7) creates a polar covalent bond where electrons are shared unequally. When the difference is very small or zero, the result is a nonpolar covalent bond, meaning electrons are shared equally between the two atoms.
The Polarity of H₂
The hydrogen molecule (H₂) is a simple diatomic molecule composed of a single covalent bond between two identical hydrogen atoms. Since both atoms are the same, the difference in electronegativity is zero. This means the shared electron pair is not pulled toward one atom more than the other, resulting in equal sharing of the electron cloud. The H₂ molecule is symmetrical, with no partial positive or negative regions forming. Therefore, the hydrogen molecule has a net zero dipole moment, classifying it as nonpolar. All homonuclear diatomic molecules, such as O₂ and N₂, are similarly nonpolar.