The oxygen molecule (\(\text{O}_2\)) is an abundant gas, making up about 21% of Earth’s atmosphere. The two oxygen atoms are held together by a strong chemical linkage. The specific connection between the two identical oxygen atoms is a double covalent bond. This bond structure dictates the molecule’s physical properties and its ability to participate in chemical reactions.
How Covalent Bonds Form
Chemical bonds form as atoms attempt to achieve a stable electron configuration, typically by filling their outermost electron shell. This drive for stability often involves reaching eight electrons in the outer shell, known as the octet rule. The electrons occupying this outermost shell are called valence electrons, and they are involved in forming chemical connections.
When atoms have a strong tendency to hold onto their own electrons, they are unlikely to give them away. Instead of transferring electrons, characteristic of ionic bonding, these atoms share electrons. This sharing mechanism between two nonmetal atoms is defined as a covalent bond. The resulting shared pair of electrons orbits both nuclei, effectively completing the outer shell for each atom.
The number of valence electrons an atom possesses determines how many bonds it needs to form to satisfy the octet rule. For example, an atom with seven valence electrons requires one more electron, so it generally forms one covalent bond. The greater the deficit from the stable eight-electron configuration, the more electrons the atom must share, leading to the formation of multiple bonds.
Covalent bonding is the standard method by which nonmetal elements join together to create molecules. The shared electron pairs act as the molecular glue, resulting in stable compounds like water, methane, or the diatomic oxygen molecule. Understanding this fundamental sharing principle explains the specific structure of \(\text{O}_2\).
The Double Bond Structure of Oxygen
Applying the principle of covalent bonding to oxygen reveals why a double bond is necessary. Each oxygen atom possesses six valence electrons in its outer shell. Since the stable octet configuration requires eight electrons, each oxygen atom needs to gain two additional electrons.
When two oxygen atoms approach, the only way for both to satisfy their need for two extra electrons is by sharing two pairs of electrons. One shared pair constitutes a single bond, so two shared pairs form a double bond. This arrangement means that four electrons are located between the two oxygen nuclei, linking them strongly.
The resulting oxygen-oxygen double bond is stronger and shorter than a single bond would be. This double bond measures approximately 121 picometers (pm) in length and requires a bond energy of about 498 kilojoules per mole (kJ/mol) to break. This high energy requirement explains the relative stability of the oxygen molecule in the atmosphere.
The structural difference between single, double, and triple bonds relates directly to the number of shared electron pairs. A triple bond involves the sharing of three pairs of electrons, resulting in an even shorter and stronger connection than the double bond found in \(\text{O}_2\). The double bond within the oxygen molecule maximizes stability while adhering to the electron requirements of each atom.
Why Oxygen Is Nonpolar
The double covalent bond in \(\text{O}_2\) is nonpolar, a classification determined by how the shared electrons are distributed. Polarity arises when one atom has a greater attraction for the shared electrons than the other, a property called electronegativity. In such cases, the electrons spend more time near the more attractive atom, creating partial negative and positive charges.
Because the oxygen molecule is made up of two identical oxygen atoms, the electronegativity values are the same. Consequently, both atoms pull on the shared electron pairs with equal force, resulting in a symmetrical distribution of charge. The shared electrons are held precisely in the middle of the bond, preventing the formation of distinct positive or negative ends.
This balanced sharing of electrons means the \(\text{O}_2\) molecule has no overall separation of charge. Molecules with this uniform charge distribution are classified as nonpolar covalent molecules. This nonpolar nature influences how oxygen interacts with other substances, allowing it to dissolve well in nonpolar solvents but having limited solubility in polar solvents like water.