A covalent bond is a fundamental link in chemistry where atoms join together to form molecules. This attachment involves a cooperative arrangement between two or more atoms, almost exclusively nonmetals, to create a stable compound. The resulting molecules are the building blocks of most substances encountered in daily life, including water, oxygen, and all organic matter. These bonds are responsible for the strong internal structure of individual molecules, defining their shape and chemical identity.
How Covalent Bonds Form
Covalent bonds form when two atoms share valence electrons, the electrons in the outermost shell of an atom. The driving force behind this sharing is the desire for each atom to achieve a more stable electronic configuration, often satisfying the octet rule by acquiring eight valence electrons. Hydrogen is a notable exception, as it only seeks two electrons to complete its shell, known as the duplet rule.
The mechanism involves the overlap of the atomic orbitals from the two participating atoms, allowing the electrons to be simultaneously attracted to both nuclei. This shared electron cloud acts as a chemical glue, linking the atoms together into a new molecular unit. The energy released during this formation results in a more stable, lower-energy state for the resulting molecule compared to the separated atoms.
Atoms can share one, two, or three pairs of electrons, leading to the formation of single, double, or triple covalent bonds, respectively. A single bond, like in a hydrogen molecule (\(\text{H}_2\)), involves sharing one pair of electrons. A double bond, such as in oxygen gas (\(\text{O}_2\)), shares two pairs, while a triple bond, found in nitrogen gas (\(\text{N}_2\)), shares three pairs. As more electron pairs are shared, the bond becomes progressively shorter and stronger.
The Difference Between Polar and Nonpolar Bonds
The nature of a covalent bond, whether it is polar or nonpolar, depends on how evenly the electrons are shared between the two atoms. This evenness is determined by a property called electronegativity, which is an atom’s inherent power to attract a bonding pair of electrons toward itself. When two atoms of the exact same element bond, such as in a molecule of oxygen (\(\text{O}_2\)), they have identical electronegativity values.
In this scenario, the shared electrons are pulled equally by both nuclei, leading to an perfectly symmetrical distribution of charge. This equal sharing defines a nonpolar covalent bond, where no distinct positive or negative ends are created within the bond. These bonds typically occur when the difference in electronegativity between the two atoms is very small, often less than 0.4.
A polar covalent bond forms when the two atoms involved have a noticeable difference in their electronegativity values, usually ranging between 0.4 and 1.7. The more electronegative atom exerts a stronger pull on the shared electron pair, drawing the electrons closer to its nucleus. This unequal sharing creates a slight negative charge (\(\delta^-\)) on the more attractive atom and a corresponding slight positive charge (\(\delta^+\)) on the less attractive atom. This separation of charge is called a dipole, exemplified by the bonds within a water molecule (\(\text{H}_2\text{O}\)).
Physical Properties of Covalent Compounds
Covalent compounds, often referred to as molecular compounds, exhibit distinct physical properties that result from their structure. Unlike ionic compounds which form large lattices, covalent compounds exist as discrete, neutral molecules. While the bonds holding the atoms together within a single molecule are strong, the forces of attraction between separate molecules, known as intermolecular forces, are comparatively weak.
This weakness in the forces between molecules explains why many covalent substances have relatively low melting and boiling points. Less thermal energy is required to overcome these weak intermolecular attractions to change the substance from a solid to a liquid or a gas. Consequently, many common covalent compounds exist as gases, such as carbon dioxide, or liquids, like water, at room temperature.
Covalent compounds are generally poor conductors of electricity in all physical states. This lack of conductivity is because the electrons are tightly held within the shared bonds and there are no free-moving charged particles, such as ions or delocalized electrons, to carry an electrical current. The overall neutral charge of the molecules prevents the transport of electrical energy through the bulk substance.