All matter is composed of fundamental building blocks called atoms. These atoms combine in various ways, forming different types of substances. Molecular substances represent a significant category, characterized by their unique atomic arrangements and the forces that hold them together.
Defining Molecular Substances
Molecular substances are chemical compounds made up of discrete units called molecules. A molecule forms when two or more atoms are joined together. For instance, a water molecule (H₂O) consists of two hydrogen atoms bonded to one oxygen atom. These atoms within a molecule are held together by a specific type of chemical bond known as a covalent bond.
Covalent bonds form when atoms share electrons to achieve a more stable electron configuration. Atoms in a covalent bond mutually contribute electrons to form shared pairs. This sharing creates a strong attractive force that links the atoms firmly within the molecule.
Key Properties
Molecular substances exhibit several characteristic physical properties. They have lower melting and boiling points compared to other types of substances. This is because the forces between individual molecules, known as intermolecular forces, are relatively weak and require less energy to overcome for a phase change to occur. Consequently, many molecular substances exist as gases or liquids at room temperature.
Molecular substances are poor electrical conductors in both their solid and liquid states. This is due to the absence of free-moving charged particles, such as ions or delocalized electrons, which are necessary to carry an electrical current. The solubility of molecular compounds in water varies; substances with stronger intermolecular attractions, like hydrogen bonding or dipole-dipole forces, tend to be more soluble in water, while others may be insoluble.
Everyday Molecular Substances
Many common materials encountered in daily life are molecular substances. Water (H₂O) is a prominent example, composed of discrete molecules held together by covalent bonds. Carbon dioxide (CO₂) is another familiar molecular substance, made of one carbon atom covalently bonded to two oxygen atoms. The oxygen gas (O₂) that we breathe is also a molecular substance, formed by two oxygen atoms sharing electrons.
Table sugar, or sucrose (C₁₂H₂₂O₁₁), is a complex molecular substance. Each sugar molecule is a distinct unit composed of carbon, hydrogen, and oxygen atoms linked by covalent bonds. Methane (CH₄), the primary component of natural gas, is another example, where a central carbon atom is covalently bonded to four hydrogen atoms.
How Molecular Substances Differ
Molecular substances fundamentally differ from ionic and metallic substances in their bonding and structural characteristics, which in turn dictate their macroscopic properties. Ionic compounds, such as table salt (sodium chloride, NaCl), are formed when electrons are transferred from one atom to another, creating positively and negatively charged ions. These oppositely charged ions are then held together by strong electrostatic forces in a rigid, repeating crystal lattice structure, rather than discrete molecules. This strong attraction results in ionic compounds having very high melting and boiling points and being good electrical conductors when molten or dissolved in water, due to the mobility of their ions.
Metallic substances, like copper or iron, feature a unique type of bonding called metallic bonding. In metals, valence electrons are not localized to individual atoms; instead, they are delocalized and move freely throughout the entire metallic structure, often described as a “sea of electrons”. This collective sharing of electrons between positively charged metal ions is responsible for metals’ characteristic properties, including excellent electrical and thermal conductivity, malleability, and ductility. Unlike molecular substances, which consist of neutral, discrete units, both ionic and metallic substances form extended structures where individual molecules are not present.