Rules for Naming Covalent Compounds
Here are the rules for naming binary covalent compounds. A binary compound is one that consists of only two elements. The names are called systematic names.
- First, name the nonmetal furthest to the left and bottom of the periodic table by its element name.
- Second, name the other nonmetal by its element name, but shorten its name and add an -ide ending.
- Add prefixes (mono-, di-, tri-, etc.) in front of each element name to indicate the number of atoms of the element. The number prefix corresponds to the subscript in the element formula. If there is no subscript, it means there is one atom of that element and the prefix is “mono-.” However, omit the “mono-” prefix in the first element’s name (e.g., CCl4 is carbon tetrachloride and not monocarbon tetrachloride).
Choosing the order of elements based on their position on the periodic table is the shorthand method of selecting elements based on their electronegativity. The element with the lower electronegativity (or higher electropositivity) is the cation or first part of the compounds, while the element with the higher electronegativity is the anion or second part of the compound. If you have trouble determining the cation and anion, you can consult a list of element electronegativity values.
The rules seem simple enough, but of course there are special cases and exceptions. Binary covalent compounds containing oxygen are “element oxide,” regardless of the electronegativity or location of the other element on the periodic table. So, a compound of chlorine and oxygen would be ClO2 and not O2Cl, even though oxygen is to the left of chlorine on the periodic table. Binary compounds of oxygen and fluoride are the exception. These are named oxygen fluorides.
Even though the element hydrogen is at the top of the periodic table, it is rarely written first in a covalent compound name or formula. Exceptions include water (H2O) and covalent compounds formed between hydrogen and a halogen (HCl, HBr, HI, etc.).
Also, some chemicals are known by their common names more often than by their systematic names. A good example is H2O, which you probably call water and not dihydrogen monoxide. Other examples are ammonia (NH3) and methane (CH4).
Covalent Compound Prefixes
- Don’t use the mono- prefix for the first element in a name.
- Drop the “o” in mono- or the “a” in tetra-, penta-, etc. for oxygen (e.g., monoxide not monooxide, pentoxide not pentaoxide)
|Number of Atoms in Compound||Element Prefix|
Examples of Covalent Compound Names
PCl5 – phosphorus pentachloride
SO2 – sulfur dioxide
N2O5 – dinitrogen pentoxide
H2O – dihydrogen monoxide
CF4 – carbon tetrafluoride
SO3 – sulfur trioxide
NO2 – nitrogen dioxide
IF7 – iodine heptafluoride
SF6 – sulfur hexafluoride
SeO – selenium monoxide
BrF5 – bromine pentafluoride
CO – carbon monoxide
S2F2 – Disulfur difluoride
Writing Binary Covalent Compound Formulas
If you’re given the systematic name of a binary covalent compound, use the same rules to write the molecular formula. Just remember that if a prefix is missing, it means there is only one atom of the element. Omit the subscript (e.g., H2O not H2O1). If necessary, review the element symbols.
- Campbell, Neil A.; Williamson, Brad; Heyden, Robin J. (2006). Biology: Exploring Life. Boston, MA: Pearson Prentice Hall. ISBN 0-13-250882-6.
- IUPAC. 1993. A Guide to IUPAC Nomenclature of Organic Compounds (the “Blue Book”). Oxford: Blackwell Scientific Publications. ISBN 0-632-03488-2.
- Rigaudy, J.; Klesney, S. P., eds. (1979). Nomenclature of Organic Chemistry. IUPAC/Pergamon Press. ISBN0-08022-3699.