A binary compound is a chemical substance that consists of exactly two different elements. The term “binary” refers only to the number of types of atoms involved, not the total number of atoms or the complexity of the molecule. Binary compounds demonstrate how elements combine to create new materials with properties distinctly different from their constituent parts.
Defining the Binary Structure
The defining feature of a binary compound is its composition: it must contain atoms from only two elements, regardless of how many atoms of each element are present in the molecule. Water ($\text{H}_2\text{O}$) is a classic example, as it contains only hydrogen and oxygen atoms, even though there are three atoms total. Similarly, hydrogen peroxide ($\text{H}_2\text{O}_2$) is also a binary compound because it consists solely of hydrogen and oxygen atoms, despite having a different ratio than water. A compound like sulfuric acid ($\text{H}_2\text{SO}_4$) is not binary because it incorporates three different elements: hydrogen, sulfur, and oxygen.
Every pure chemical compound, including binary ones, adheres to the Law of Definite Proportions. This law dictates that the constituent elements within a compound are always present in a fixed ratio by mass. For example, carbon dioxide ($\text{CO}_2$) always has one carbon atom for every two oxygen atoms, establishing a consistent whole-number ratio that governs its chemical identity.
Classification by Bond Type
Binary compounds are broadly categorized based on the specific mechanism through which their two elements bond, which fundamentally dictates the compound’s physical and chemical properties. This distinction centers on whether electrons are transferred from one atom to another or are shared between them. The two resulting categories are ionic and covalent binary compounds.
Ionic Binary Compounds
Ionic binary compounds form when a metal atom combines with a nonmetal atom, typically involving elements from opposite sides of the periodic table. This combination occurs through the transfer of one or more valence electrons from the metal to the nonmetal. The metal loses electrons to become a positively charged ion (cation), while the nonmetal gains electrons to become a negatively charged ion (anion).
The resulting compound is held together by a strong electrostatic attraction between these oppositely charged ions, which constitutes the ionic bond. Sodium chloride ($\text{NaCl}$), commonly known as table salt, is a prime example. Because of this strong attraction, ionic compounds generally exist as crystalline solids with high melting points.
Covalent (Molecular) Binary Compounds
Covalent binary compounds, also called molecular compounds, are formed when two nonmetal atoms combine. Unlike ionic compounds, the atoms in a covalent compound share electrons rather than transferring them. This sharing of electrons creates a covalent bond, which holds the atoms together to form a discrete molecule.
These compounds can involve two atoms of the same element, such as oxygen gas ($\text{O}_2$), or two different nonmetals, such as carbon dioxide ($\text{CO}_2$) or phosphorus trichloride ($\text{PCl}_3$). Covalent compounds generally exhibit lower melting and boiling points compared to ionic compounds because the intermolecular forces holding the molecules to one another are much weaker than the electrostatic forces in an ionic lattice.
Rules for Naming Binary Compounds
The systematic naming of a binary compound depends entirely on its bond type, with separate rules for ionic and covalent structures. This dual system ensures clarity and communicates information about the compound’s composition and bonding characteristics.
The nomenclature for ionic binary compounds involves naming the cation first, followed by the anion, which always takes the element root and the suffix “-ide.” For metals that form only one stable ion (like those in Groups 1 and 2), the name is straightforward, such as potassium sulfide ($\text{K}_2\text{S}$). No prefixes are used in naming ionic compounds, as the charges dictate the combining ratio.
Type II ionic compounds involve metals, such as transition metals, that can form multiple ions with different charges. In these cases, the Stock system requires a Roman numeral in parentheses immediately following the metal’s name to indicate its charge. For example, $\text{FeO}$ and $\text{Fe}_2\text{O}_3$ are distinguished as iron(II) oxide and iron(III) oxide, respectively.
Covalent binary compounds use a system that relies on Greek prefixes to indicate the number of atoms of each element in the molecule. The first element is named using its full name. Prefixes like di-, tri-, or tetra- are used if more than one atom is present, though “mono-” is generally omitted for the first element, as seen in carbon monoxide ($\text{CO}$). The second element is named with a prefix indicating its quantity, followed by the “-ide” suffix, such as dinitrogen tetroxide ($\text{N}_2\text{O}_4$).