Matter is built from fundamental chemical compounds. These compounds form when atoms of different elements chemically combine to create new substances with unique properties. Chemical bonding holds these atoms together, resulting in several types of compounds. Among the most prevalent are molecular compounds, which form the basis for countless substances encountered daily in nature and life.
Defining Molecular Compounds
A molecular compound is a substance created when two or more nonmetal atoms join together through a chemical connection. The defining characteristic is that they exist as discrete, neutral units called molecules. Each molecule acts as an independent entity, possessing all the chemical and physical characteristics of the substance. For instance, a single water molecule is the smallest unit that still behaves as water.
Molecular compounds are restricted to nonmetal elements, such as carbon, oxygen, hydrogen, and nitrogen. This contrasts with ionic compounds, which typically involve a metal and a nonmetal. Unlike a crystal lattice, which is an extended network of atoms, molecular compounds are finite groups of atoms bound tightly together. Weaker forces act between one molecule and the next. The chemical formula of a molecular compound, such as CO2, indicates the exact number of atoms of each element contained within that single molecule.
The Mechanics of Covalent Bonding
The formation of a molecular compound is driven by covalent bonding. This bond is the mechanism by which atoms share valence electrons—the electrons in the outermost shell—to achieve greater stability. Most atoms aim to satisfy the Octet Rule, which states that atoms are most stable when surrounded by eight valence electrons, mimicking noble gases. Hydrogen is an exception, requiring only two electrons for stability.
In a covalent bond, the electron pair is mutually attracted to the nuclei of both atoms, effectively linking them. This sharing allows each participating atom to count the shared electrons toward its stable electron count. The bond forms as the two atomic orbitals overlap, minimizing the system’s overall energy. The bond’s strength and length depend directly on the number of electron pairs shared between the nonmetal atoms.
A single covalent bond involves sharing one pair of electrons (two electrons), such as the bond between two hydrogen atoms in an H2 molecule. Atoms can form multiple bonds when they need to share more electrons to fulfill the Octet Rule. A double bond consists of two shared electron pairs (four electrons), as found in an O2 molecule. The strongest and shortest type is a triple bond, formed by sharing three electron pairs (six electrons), as seen in the nitrogen molecule, N2.
Physical Characteristics and Common Examples
The discrete nature of molecules and covalent bonding result in distinctive physical properties. Within each molecule, covalent bonds are strong, but the forces of attraction between separate molecules, known as intermolecular forces, are much weaker. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds. Because these weak forces are easily disrupted, molecular compounds generally have low melting and boiling points.
Many molecular compounds exist as gases or liquids at room temperature, or as soft solids that melt easily. Water (H2O) is a liquid, and carbon dioxide (CO2) is a gas, illustrating this tendency. Molecular compounds are poor conductors of electricity in any state, including when dissolved in water. This is because they are electrically neutral and do not dissociate into charged particles or ions that can carry an electrical current.
Common Examples
Common examples of molecular compounds are abundant. Water is the most recognized example, composed of two hydrogen atoms and one oxygen atom. Table sugar, or sucrose (C12H22O11), is a larger molecular compound formed entirely from nonmetals carbon, hydrogen, and oxygen. Carbon dioxide, the gas used by plants for photosynthesis, is another classic example.