Covalent Nomenclature
Covalent nomenclature is the systematic method chemists use to name binary covalent (molecular) compounds — molecules formed when two nonmetal elements share electrons. Unlike ionic compounds, which are named by identifying the cation and anion, covalent compounds require Greek numerical prefixes to specify exactly how many atoms of each element are present in one molecule. This precision is essential because two nonmetals can often combine in multiple different ratios, each producing a chemically distinct substance with different properties. For example, nitrogen and oxygen can form at least five different compounds: NO, NO₂, N₂O, N₂O₃, and N₂O₅. Without a clear naming system, communication between chemists would be impossible. The IUPAC (International Union of Pure and Applied Chemistry) system for binary covalent compounds solves this problem by attaching prefixes such as mono-, di-, tri-, and tetra- directly to the element names to indicate atom counts. Mastering this system allows you to both name any binary molecular formula you are given and write the correct formula from any name. A related naming situation arises when certain binary covalent compounds — specifically those where hydrogen is bonded to a nonmetal — are dissolved in water. These solutions often behave as acids, and they follow a distinct set of naming rules involving the prefix 'hydro-' and the suffix '-ic acid.' Understanding both naming conventions — for pure binary covalent molecules and for their aqueous acid solutions — forms the foundation of chemical communication in molecular chemistry.
- Identify the Two Nonmetal Elements
- Apply Greek Prefixes to Indicate Atom Count
- Name the First Element
- Name the Second Element with the '-ide' Suffix
- Naming Binary Acids (aqueous HX solutions)
- Converting Names Back to Formulas
Identify the Two Nonmetal Elements
Binary covalent compounds consist of exactly two different nonmetal elements. Common examples include combinations of C, N, O, P, S, F, Cl, Br, and I. The element listed first in the formula is named first; it is usually the less electronegative element (further left or lower on the periodic table). Example: In PCl₃, phosphorus (P) is named before chlorine (Cl).
Apply Greek Prefixes to Indicate Atom Count
Attach a Greek prefix to each element name to show how many atoms of that element appear in the formula. The prefixes are: 1 = mono, 2 = di, 3 = tri, 4 = tetra, 5 = penta, 6 = hexa, 7 = hepta, 8 = octa, 9 = nona, 10 = deca. Important rule: the prefix 'mono-' is NEVER used for the first element. It is used for the second element when there is only one of it (e.g., carbon monoxide, CO). When a prefix ending in 'a' or 'o' is placed before a vowel-starting element name, the final vowel of the prefix is dropped for easier pronunciation (e.g., tetra + oxide = tetroxide, not tetraoxide).
Name the First Element
Write the name of the first element with its appropriate prefix (omitting mono-). Use the element's full name. Examples: N₂O₄ — first element is N (nitrogen), there are 2 atoms, so: 'dinitrogen'. SF₆ — first element is S (sulfur), there is 1 atom, so: 'sulfur' (no mono-).
Name the Second Element with the '-ide' Suffix
Write the second element's name with the appropriate prefix and change its ending to '-ide'. You drop the element's normal suffix and replace with '-ide'. Examples: oxygen → oxide, fluorine → fluoride, chlorine → chloride, sulfur → sulfide, nitrogen → nitride, bromine → bromide. So N₂O₄ becomes 'dinitrogen tetroxide' and SF₆ becomes 'sulfur hexafluoride.'
Naming Binary Acids (aqueous HX solutions)
When a hydrogen-nonmetal binary compound (HX) is dissolved in water, it may act as an acid. These solutions are named differently from the pure gas-phase compound. Rule: Add the prefix 'hydro-' to the nonmetal stem, then add the suffix '-ic acid,' and write 'acid' at the end. The formula is always written with (aq) to show it is in aqueous solution. Examples: HCl(g) is 'hydrogen chloride' (pure compound), but HCl(aq) is 'hydrochloric acid.' HBr(aq) = hydrobromic acid. HF(aq) = hydrofluoric acid. H₂S(aq) = hydrosulfuric acid. Note: HF(aq) is a weak acid despite following this naming pattern.
Converting Names Back to Formulas
To write a formula from a name, reverse the process: identify the prefixes to determine subscripts, identify the element symbols from the names, and write the first-named element first. Example: 'diphosphorus pentasulfide' → P₂S₅. 'Carbon tetrachloride' → CCl₄ (mono- is implied for carbon even though not stated). 'Dinitrogen monoxide' → N₂O.
The Key Equation
- \ce{N2} — Dinitrogen (mol): Elemental nitrogen gas; two nitrogen atoms triple-bonded together.
- \ce{O2} — Dioxygen (mol): Elemental oxygen gas; two oxygen atoms double-bonded together.
- \ce{NO2} — Nitrogen dioxide (mol): A binary covalent compound; one nitrogen atom bonded to two oxygen atoms. Named using the prefix mono- (implied for N) and di- for two oxygens, plus the '-ide' suffix changed to 'oxide'.
Worked Example
Problem. Name the following compounds and write the formula for 'diphosphorus pentoxide': (a) CCl₄, (b) N₂O₃, (c) HBr(aq).
Answer. (a) carbon tetrachloride; (b) dinitrogen trioxide; (c) hydrobromic acid; diphosphorus pentoxide = P₂O₅
Origins · 1 minJöns Jacob Berzelius and how this was discovered
Berzelius developed the modern chemical symbol system still used today, assigning one- or two-letter abbreviations to elements based on their Latin names. He also introduced the use of subscript numbers to show the number of atoms of each element in a compound's formula — the very notation that makes covalent nomenclature possible. Without his systematic approach to chemical notation, writing and naming compounds like N₂O₅ or PCl₃ would be far more ambiguous and cumbersome. Berzelius discovered three chemical elements — selenium, thorium, and cerium — and first isolated silicon in pure form. He was also the first chemist to use the terms 'organic' and 'inorganic' to distinguish carbon-based compounds from minerals.
Related Concepts
- Ionic Nomenclature — Ionic nomenclature uses different rules (no prefixes, Roman numerals for transition metals) compared to covalent nomenclature, but both systems rely on the '-ide' suffix for binary anions and simple binary formulas.
- Covalent Bonding — Understanding how nonmetals share electrons to form covalent bonds explains why binary covalent compounds exist in so many ratios, making a precise naming system necessary.
- Molecular Geometry (VSEPR) — Once you can name and write formulas for covalent compounds, the next step is predicting their 3D shapes using VSEPR theory, which depends on knowing the exact number of bonding and lone-pair electrons.
- Introduction to Acids and Bases — Binary acid naming (hydro-__-ic acid) is the first step into acid-base chemistry; understanding what makes HCl(aq) an acid connects covalent nomenclature to the Arrhenius and Brønsted-Lowry definitions.