Difficulty: Introductory | Prerequisites: Know the difference between metals and nonmetals; familiarity with ionic naming helps for contrast.
Covalent (molecular) compounds are formed between two nonmetals sharing electrons, rather than transferring them. Because nonmetals can combine in many different ratios (CO and CO₂ are both real compounds), you cannot rely on charge balancing to figure out the formula. Instead, the naming system uses Greek prefixes to state exactly how many atoms of each element are present. This is the third and final naming system covered in introductory inorganic nomenclature, alongside binary ionic and transition-metal ionic. If you can classify a compound as covalent (two nonmetals, no metal), the prefix system is the one to reach for.
Two nonmetals together means covalent naming. Use Greek prefixes (mono-, di-, tri-, tetra-, penta-, hexa-) to say how many atoms of each element are in the formula, and change the second element's ending to "-ide." Drop "mono-" on the first element only.
Covalent (molecular) compound
A compound formed by two or more nonmetals sharing electrons. Think of it as atoms cooperating rather than one robbing the other.
Greek prefixes
The number prefixes used in covalent naming to indicate how many atoms of each element are present:
1 = mono-
2 = di-
3 = tri-
4 = tetra-
5 = penta-
6 = hexa-
7 = hepta-
8 = octa-
9 = nona-
10 = deca-
"-ide" suffix (covalent context)
Same rule as ionic naming: the second element's ending changes to "-ide." Oxygen becomes oxide, fluorine becomes fluoride, chlorine becomes chloride, sulfur becomes sulfide.
"Mono-" rule
The prefix "mono-" is dropped from the first element but kept on the second. CO is "carbon monoxide," not "monocarbon monoxide." CO₂ is "carbon dioxide," not "monocarbon dioxide."
Check that both elements are nonmetals. If one is a metal, use ionic naming instead.
Name the first element with its Greek prefix (unless the prefix would be "mono-," in which case drop it).
Name the second element with its Greek prefix, then change its ending to "-ide."
Watch for vowel dropping: "mono-oxide" becomes "monoxide" (drop the "o" before "oxide"); "penta-oxide" becomes "pentoxide" (drop the "a" before "oxide"). This is just for pronunciation flow.
The prefix tells you the subscript directly. Diiodine pentoxide: 2 iodines, 5 oxygens, so I₂O₅.
No charge balancing is needed. The prefixes carry all the information.
Naming from formula:
I₂O₅: 2 iodines, 5 oxygens. Diiodine pentoxide.
SO₃: 1 sulfur, 3 oxygens. Sulfur trioxide (no "mono-" on sulfur).
BrF₅: 1 bromine, 5 fluorines. Bromine pentafluoride.
B₂Cl₄: 2 borons, 4 chlorines. Diboron tetrachloride.
P₂S₅: 2 phosphoruses, 5 sulfurs. Diphosphorus pentasulfide.
PBr₅: 1 phosphorus, 5 bromines. Phosphorus pentabromide.
NF₄: 1 nitrogen, 4 fluorines. Nitrogen tetrafluoride.
CI₄: 1 carbon, 4 iodines. Carbon tetraiodide.
SCl₄: 1 sulfur, 4 chlorines. Sulfur tetrachloride.
Writing formula from name:
Carbon dioxide: 1 carbon, 2 oxygens. CO₂.
Xenon hexafluoride: 1 xenon, 6 fluorines. XeF₆.
Nitrogen trichloride: 1 nitrogen, 3 chlorines. NCl₃.
Boron tribromide: 1 boron, 3 bromines. BBr₃.
Dichlorine monoxide: 2 chlorines, 1 oxygen. Cl₂O.
Students sometimes use Greek prefixes on ionic compounds. NaCl is "sodium chloride," not "sodium monochloride." Prefixes are for covalent compounds only.
Students sometimes use Roman numerals on covalent compounds. SO₃ is "sulfur trioxide," not "sulfur (VI) oxide." Roman numerals are for transition-metal ionic compounds.
Forgetting to drop "mono-" on the first element. "Monocarbon dioxide" is not correct; it is simply "carbon dioxide."
Misspelling prefix-vowel combinations. It is "pentoxide" (not "pentaoxide") and "monoxide" (not "monooxide"). The trailing vowel of the prefix drops when the element name starts with "o" or "a."
⚠️ The most common exam question type: a mixed list of formulas where you must first classify each compound (ionic vs. covalent) and then apply the correct naming system. If you misclassify, the entire name is wrong.
⚠️ Know all Greek prefixes through at least deca- (10). Penta- and hexa- appear frequently in worksheet and exam problems.
⚠️ Vowel-dropping rules (pentoxide, monoxide) are tested explicitly on some exams. If you write "pentaoxide," expect to lose the mark.
True or False: N₂O₄ is named "dinitrogen tetraoxide" (not "tetroxide"). (Answer: True. The "a" in "tetra" does not drop before "o" in this case per IUPAC convention, though some textbooks accept "tetroxide." Follow your professor's preference.)
Fill in the blank: The Greek prefix for 5 is ____. (Answer: penta-.)
True or False: CO₂ is named "monocarbon dioxide." (Answer: False. Drop "mono-" on the first element. It is carbon dioxide.)
Fill in the blank: The formula for diphosphorus pentasulfide is ____. (Answer: P₂S₅.)
True or False: Covalent compound names use Roman numerals. (Answer: False. They use Greek prefixes.)
Q: Name the compound BrF₅.
A: One bromine, five fluorines. Bromine pentafluoride.
Q: Write the formula for dichlorine monoxide.
A: Di- = 2 chlorines, mono- = 1 oxygen. Formula: Cl₂O.
Q: Name the compound B₂Cl₄.
A: Two borons, four chlorines. Diboron tetrachloride.
Q: A student names SO₃ as "sulfur (VI) oxide." What is wrong?
A: SO₃ is a covalent compound (two nonmetals), so it uses Greek prefixes, not Roman numerals. The correct name is sulfur trioxide.
Q: Write the formula for xenon hexafluoride.
A: Hexa- = 6 fluorines. Formula: XeF₆.
Q: How do you decide whether a compound uses Greek prefixes or Roman numerals?
A: Look at the elements. Two nonmetals = covalent, use Greek prefixes. A metal and a nonmetal = ionic; if the metal has variable charges (transition metal), use Roman numerals.
Covalent naming is the counterpart to ionic naming. The first step in any nomenclature problem is classifying the compound, so understanding both systems and when each applies is the real skill being tested.
This connects forward to molecular geometry (VSEPR theory), where you will draw the shapes of these molecules. Knowing the formula from the name is the starting point for drawing a Lewis structure.
Some covalent compounds have common names that override the systematic name: H₂O is "water" (not "dihydrogen monoxide" in normal usage), and NH₃ is "ammonia." Your professor will note which common names are expected.
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