A covalent bond forms when two nonmetal atoms share electrons to achieve a stable electronic configuration, typically resulting in a discrete molecule. Unlike ionic compounds, two nonmetals can often combine in multiple proportions to form entirely different substances. For instance, carbon and oxygen can form both carbon monoxide (\(\text{CO}\)) and carbon dioxide (\(\text{CO}_2\)). This variability makes a systematic naming system, known as nomenclature, necessary for clear communication. The system for binary covalent compounds—those made of only two different elements—utilizes Greek numerical prefixes to unambiguously specify the exact number of atoms of each element present in the molecule.
The Core Components of Covalent Naming
Naming binary covalent compounds relies on two fundamental linguistic adjustments. The first uses numerical prefixes to indicate the number of atoms for each element in the molecular formula. This feature is unique to covalent nomenclature, as it directly conveys the compound’s molecular composition.
The second involves modifying the name of the second element listed in the formula. This element always has its name shortened to its root and is given the suffix “-ide.” For example, oxygen becomes oxide, chlorine becomes chloride, and sulfur becomes sulfide.
The element that appears first in the chemical formula is typically the one less electronegative and retains its full elemental name.
Decoding the Numerical Prefixes
The numerical prefixes indicate how many atoms of a specific element are present in the molecule.
- One atom: mono-
- Two atoms: di-
- Three atoms: tri-
- Four atoms: tetra-
- Five atoms: penta-
- Six atoms: hexa-
- Seven atoms: hepta-
- Eight atoms: octa-
- Nine atoms: nona-
- Ten atoms: deca-
Applying the Rules A Step-by-Step Guide
The process for naming a binary covalent compound involves systematically applying the prefixes and suffixes to the elements listed in the chemical formula. The first step is to name the first element in the formula using its full elemental name. Next, count the number of atoms for that first element and attach the corresponding numerical prefix.
The prefix mono- is almost always omitted for the first element in the compound. For instance, the compound \(\text{CO}\) is named Carbon Monoxide, not Monocarbon Monoxide.
The next step focuses on the second element in the formula, which requires its own numerical prefix based on its atomic count. This prefix is then followed by the root of the second element’s name, which must always end with the “-ide” suffix. For example, the compound \(\text{PCl}_5\) contains one phosphorus atom and five chlorine atoms, leading to the name Phosphorus Pentachloride.
A linguistic adjustment, the Vowel Drop Rule, sometimes applies to prefixes ending in ‘a’ or ‘o’ when the second element starts with a vowel, most often oxygen. For example, in the compound \(\text{N}_2\text{O}_4\), the name is Dinitrogen Tetroxide, where the ‘a’ from the prefix tetra- is dropped for easier pronunciation.
Applying these steps allows for the unambiguous naming of molecules. For \(\text{CO}_2\), the single carbon atom is left without a prefix, and the two oxygen atoms take the di- prefix and the -ide suffix, resulting in Carbon Dioxide. Similarly, \(\text{N}_2\text{O}_4\) uses di- for the two nitrogen atoms and tetra- combined with the vowel drop for the four oxygen atoms, yielding Dinitrogen Tetroxide.