Chemical nomenclature provides a standardized language for discussing substances. When chemists communicate about a specific molecule, they rely on systematic rules to ensure clarity and accuracy. These rules differ significantly depending on the type of chemical bond.
Compounds formed by the sharing of electrons, called covalent or molecular compounds, require a specific naming approach. Because covalent bonds often allow the same two elements to combine in multiple ratios, the naming system must explicitly indicate the exact number of atoms present in the molecule. This quantitative approach relies on a series of Greek numerical prefixes.
What Defines a Covalent Compound
A covalent compound is formed when two nonmetal atoms interact, resulting in a chemical bond where electrons are shared between the atoms. This sharing allows each atom to achieve a stable electron configuration, generally fulfilling the octet rule. Unlike ionic compounds, covalent bonding creates discrete molecules.
This molecular structure is why the naming system needs to be precise about the atomic ratios. The same two elements can often combine in several different ways, resulting in molecules with unique chemical and physical properties. For example, nitrogen and oxygen can form nitrogen monoxide (\(\text{NO}\)) or nitrogen dioxide (\(\text{NO}_2\)), which are two chemically distinct substances.
The numerical prefixes are incorporated into the name to act as a molecular blueprint, providing the necessary quantitative information. This system ensures that the name unambiguously corresponds to a single molecular formula.
The Standard Numerical Prefixes
The foundation of naming molecular compounds is a set of Greek-derived numerical prefixes that translate the atom count into the compound’s name. These prefixes are attached to the element names to indicate exactly how many atoms of that element are present. They serve as the primary tool for distinguishing between compounds composed of the same elements but with different ratios.
The standard prefixes used for binary covalent compounds up to ten atoms are:
- Mono- (1)
- Di- (2)
- Tri- (3)
- Tetra- (4)
- Penta- (5)
- Hexa- (6)
- Hepta- (7)
- Octa- (8)
- Nona- (9)
- Deca- (10)
These prefixes are applied regardless of the element they are modifying. For example, four oxygen atoms would be tetraoxide and five chlorine atoms would be pentachloride.
Applying the Rules for Naming Binary Compounds
Applying these prefixes involves a specific sequence of rules designed to standardize the naming of binary covalent compounds—those containing only two different elements.
Ordering and Naming Elements
The first step is determining the order of the elements. Generally, the element farther to the left or lower down on the periodic table is named first. For instance, carbon is named before oxygen in \(\text{CO}_2\).
The name of the first element is written out completely. The numerical prefix corresponding to the number of atoms must be attached to the name of the second element. The name of the second element is also modified by dropping its normal ending and replacing it with the suffix -ide.
Using the Mono- Prefix
The prefix mono- is always omitted for the first element, even if only one atom is present. For example, \(\text{CO}_2\) is named carbon dioxide, not monocarbon dioxide. The absence of a prefix implies a count of one for the first element.
If the first element has more than one atom, the appropriate prefix, such as di- or tetra-, is always included. For instance, \(\text{N}_2\text{O}_4\) is systematically named dinitrogen tetroxide, clearly indicating two nitrogen atoms and four oxygen atoms.
Phonetic Adjustments
A phonetic rule applies when combining the prefix and the element name to improve pronunciation. If the element name begins with a vowel, the final vowel (either ‘a’ or ‘o’) of the numerical prefix is usually dropped.
This is why \(\text{CO}\) is named carbon monoxide (dropping the ‘o’ from mono). This adjustment is typically observed with oxygen-containing compounds. For example, \(\text{P}_4\text{O}_{10}\) is named tetraphosphorus decoxide, where the ‘a’ from deca- is dropped before the vowel ‘o’ in oxide. This modification ensures the systematic name is easily communicated.
Key Exceptions and Common Names
While the systematic rules cover the majority of covalent compounds, several important exceptions exist that are widely recognized and accepted by the International Union of Pure and Applied Chemistry (IUPAC).
Many compounds have historically retained their non-systematic, common names due to their ubiquity and long history of use. The best-known example is water (\(\text{H}_2\text{O}\)), which is never called dihydrogen monoxide. The common name is universally understood and accepted.
Other common molecular compounds that bypass the prefix system include ammonia (\(\text{NH}_3\)) and methane (\(\text{CH}_4\)). These established names are used universally in place of their technically correct systematic names, such as nitrogen trihydride or carbon tetrahydride. These exceptions illustrate the balance between systematic nomenclature and practical usage.