Covalent bonding is a fundamental force in chemistry, responsible for forming most molecules. This type of chemical bond occurs when atoms share one or more pairs of their outermost electrons. The sharing results in a stable molecular unit where the participating atoms are held tightly together. This process is the primary mechanism by which non-metal atoms combine, creating everything from simple gases to complex organic molecules like DNA.
Why Atoms Seek Covalent Bonds
Atoms engage in bonding because they are more stable when their outermost electron shell, the valence shell, is completely filled. For most atoms, maximum stability is achieved by possessing eight electrons in the valence shell, known as the octet rule. Atoms seek an electron configuration similar to noble gases, which are chemically non-reactive due to their full shells.
The duet rule is a notable exception, applying to the lightest atoms like hydrogen and helium. These atoms only require two electrons to complete their first and only shell. By sharing electrons, atoms count the shared electrons toward the full shell of both participating partners simultaneously. This movement toward a lower-energy, more stable electron configuration drives covalent bond formation.
The Process of Electron Sharing
The sharing of electrons during covalent bonding involves the precise physical phenomenon of atomic orbital overlap. An atomic orbital represents the region of space where an electron is most likely to be found around a single nucleus. When two atoms approach each other, their respective atomic orbitals begin to overlap in the region between the two nuclei.
This overlap leads to constructive interference between the electron waves, resulting in a significant increase in electron density located between the two atomic nuclei. The shared electron pair is simultaneously attracted to the positive charge of both nuclei, creating a strong electrostatic force that holds the molecule together. The greater this orbital overlap, the stronger the resulting covalent bond will be. The electrons are delocalized into a shared molecular orbital that encompasses both atoms.
Variations in Electron Sharing
Covalent bonds are classified based on the quantity of electron pairs that are shared between the two atoms, affecting the bond’s physical properties. A single bond involves the sharing of just one pair of electrons, as seen in a molecule of hydrogen gas (H₂). This is the longest and weakest type of covalent connection between a given pair of atoms.
When atoms need to share more electrons to satisfy their octet, they form multiple bonds. A double bond involves sharing two pairs of electrons (four total), such as in oxygen gas (O₂). A triple bond involves sharing three pairs of electrons (six total), commonly seen in nitrogen gas (N₂). As the number of shared electron pairs increases, the attractive force between the nuclei intensifies. Triple bonds are the shortest and strongest, with double bonds falling in the intermediate range for length and strength.
Unequal Sharing and Polarity
While “sharing” suggests an equal distribution of electrons, this is often not the case in covalent bonds between different elements. Electronegativity describes an atom’s inherent power to attract a shared pair of electrons toward itself within a bond. When two atoms of the same element bond, their identical electronegativity results in equal sharing, forming a nonpolar covalent bond.
When atoms with differing electronegativity values bond, the sharing becomes unequal, creating a polar covalent bond. The electron pair spends more time closer to the nucleus of the more electronegative atom. This uneven distribution of negative charge causes the more attractive atom to develop a slight negative charge (δ⁻), while the less attractive atom develops a slight positive charge (δ⁺).
A classic example is the water molecule (H₂O), where the oxygen atom has a much higher electronegativity than the two hydrogen atoms. Oxygen pulls the shared electrons strongly toward itself, leading to a partial negative charge on the oxygen side of the molecule. The hydrogen atoms, having less electron density around them, develop partial positive charges. This molecular polarity grants water its unique property as an excellent solvent.