Molecular oxygen (\(\text{O}_2\)) is essential for cellular respiration. While biologically important, the reason for its nonpolar nature is straightforward. A polar molecule has an uneven distribution of electrical charge, creating positive and negative ends. \(\text{O}_2\), however, maintains a perfectly balanced charge distribution, which governs its behavior in the body and the environment.
Defining the Force: Electronegativity
Polarity in a chemical bond is tied to electronegativity, which is an atom’s ability to attract shared electrons in a covalent bond toward itself. Atoms with higher electronegativity exert a stronger pull on the electron cloud. The Pauling scale quantifies this pull, with values typically ranging from 0.7 to 4.0.
The difference in electronegativity determines the bond type. A large difference (greater than 1.8) often results in an ionic bond, where electrons are transferred. A moderate difference (0.4 to 1.8) creates a polar covalent bond, where electrons are shared unequally, leading to partial charges. When the difference is very small or exactly zero, the electrons are shared equally, forming a nonpolar covalent bond.
Perfect Balance: The Symmetrical Structure of O2
Molecular oxygen (\(\text{O}_2\)) is a diatomic molecule consisting of two oxygen atoms joined by a double covalent bond. Since both atoms are identical, they share the exact same electronegativity value (3.44 on the Pauling scale). When they bond, the difference in electronegativity is precisely zero.
This zero difference ensures the shared electrons are pulled equally by both nuclei, preventing any separation of charge or creation of partial poles. The absence of charge separation results in a net dipole moment of zero for the \(\text{O}_2\) molecule. A dipole moment measures overall polarity, and a zero value confirms a nonpolar structure.
Practical Impact: Why O2’s Nonpolarity Matters
The nonpolar nature of \(\text{O}_2\) has consequences for its behavior in biological systems, especially its interaction with water. The rule “like dissolves like” means that since water is a highly polar solvent, nonpolar \(\text{O}_2\) has very low solubility in it. This low solubility requires organisms to use specialized transport mechanisms.
Only about 1.5% to 2% of oxygen in the blood is dissolved directly in the water-based plasma. The remaining 98% must be carried by the protein hemoglobin, which is packaged inside red blood cells. Hemoglobin is a dedicated carrier that binds to \(\text{O}_2\), compensating for its poor solubility in the watery environment of blood. Without this specialized protein, the body could not deliver enough oxygen to meet metabolic demands.