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General Chemistry I · Atoms, Molecules & Ions

Naming Molecular (Covalent) Compounds

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Molecular (covalent) compounds form between two or more nonmetals, which share electrons rather than transferring them. Because the same pair of nonmetals can combine in several different ratios (carbon monoxide CO vs. carbon dioxide CO₂; nitrogen monoxide NO vs. nitrogen dioxide NO₂), the name must specify how many of each atom are present. Molecular nomenclature does this with Greek prefixes (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-) attached to each element name, with the second element ending in -ide. The prefixes remove all ambiguity — a feature the metal–nonmetal naming system does not need, since variable charges there are handled by Roman numerals instead.

Why this matters

Molecular compounds include water, carbon dioxide, sugars, and most of organic chemistry and biochemistry. Their names encode exact composition, which is why "carbon monoxide" (CO, a deadly gas) and "carbon dioxide" (CO₂, harmless at normal levels) can never be confused — a difference of one oxygen atom changes both the name and the danger. Mastering prefixes is also the gateway to naming larger organic and inorganic molecules systematically.

The college version

Key Ideas

  • Used for two nonmetals (e.g., N₂O, CO₂, PCl₃, SF₆).
  • Prefixes count atoms: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10).
  • Second element takes the "-ide" ending (oxygen → oxide, chlorine → chloride, nitrogen → nitride).
  • "Mono-" is omitted on the first element (CO is carbon monoxide, not monocarbon monoxide).
  • "Mono-" is kept on the second element when there is exactly one (CO = carbon monoxide).
  • When the prefix ends in "a" or "o" and the element begins with a vowel, the prefix's final vowel is often dropped (mono + oxide → monoxide; penta + oxide → pentoxide).
  • No Roman numerals, no charges — those belong to ionic compounds.
  • Hydrogen compounds have common names too (H₂O water, NH₃ ammonia, CH₄ methane) that override the prefix system.

Equations and Variables

This topic is qualitative; the "formula" to remember is the name pattern:

(prefix₁)(element₁) + (prefix₂)(element₂ + "-ide")

with the drop-mono-on-first and vowel-elision rules noted above. The prefixes themselves are the 1–10 counting words.

How It Works or Problem-Solving Method

Naming a formula:

  1. Confirm both elements are nonmetals (use prefixes; if a metal is present, switch to ionic rules).
  2. Count each element's atoms from its subscript.
  3. Attach the Greek prefix to each element (drop "mono-" on the first element).
  4. Change the second element to its "-ide" stem.

Writing a formula from a name:

  1. Read each prefix as a subscript.
  2. The first element keeps its symbol with that subscript; the second element's "-ide" stem tells you the element, with the prefix as its subscript.
  3. If the first element has no prefix, its subscript is 1 (omitted).

Worked Example

Problem 1: Name N₂O₄ and PCl₃.

N₂O₄: two nitrogen, four oxygen → dinitrogen tetroxide ("tetra-" drops its final "a" before "oxide"). PCl₃: one phosphorus (no "mono-"), three chlorine → phosphorus trichloride.

Problem 2: Name CO and CO₂.

CO: one carbon (no prefix), one oxygen → carbon monoxide ("mono-" kept on oxygen). CO₂: one carbon, two oxygen → carbon dioxide.

Problem 3: Write formulas for sulfur hexafluoride and dinitrogen monoxide.

Sulfur hexafluoride → S + 6 F = SF₆. Dinitrogen monoxide → N₂ + 1 O = N₂O.

Problem 4: Write the formula for tetraphosphorus decoxide and name it with the elision rule applied.

Tetraphosphorus decoxide → P₄O₁₀ (deca- + oxide → "decoxide", with the "a" dropped). Commonly written P₄O₁₀.

Common Confusions

  • "CO should be 'monocarbon monoxide.'" Wrong — "mono-" is dropped on the first element, so it's simply carbon monoxide.
  • "CO₂ is 'carbon oxide.'" Wrong — the "di-" on oxygen is essential; dropping it would describe a different compound.
  • "Molecular compounds use Roman numerals." Wrong — Roman numerals denote ionic charges (iron(III) chloride); molecular compounds use prefixes (phosphorus trichloride).
  • "N₂O₄ is 'dinitrogen tetraoxide.'" Wrong — the prefix's final vowel is elided before a vowel: tetroxide, not "tetraoxide."
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Molecular naming is like counting blocks in a LEGO model and announcing the number out loud: "this model has two red blocks and four blue blocks" → "dinitrogen tetroxide." The Greek prefixes are just the numbers 1 through 10 in disguise — mono, di, tri, tetra, penta — so "carbon dioxide" literally means "one carbon, two oxygens." The only quirk is that we skip saying "one" for the first block (nobody says "monocarbon"), but we do say it for the second. Limit of the analogy: LEGO blocks can be clicked together and apart freely, but real atoms bond with fixed, specific ratios — the name describes the exact molecule, not an arbitrary build.

Key takeaways

  • Prefixes: mono(1), di(2), tri(3), tetra(4), penta(5), hexa(6), hepta(7), octa(8), nona(9), deca(10).
  • Second element ends in -ide.
  • Drop "mono-" on the first element only.
  • Keep "mono-" on the second element (CO = carbon monoxide).
  • Elide final "a"/"o" before a vowel: monoxide, tetroxide, pentoxide.
  • Used for nonmetal + nonmetal only; metals → ionic rules.
  • H₂O, NH₃, CH₄ use common names, not prefixes.
  • Molecular compounds = two nonmetals, named with Greek prefixes.
  • Prefixes: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca.
  • Second element ends in -ide.
  • Drop "mono-" on the first element; keep it on the second.
  • Elide final vowel before a vowel (monoxide, tetroxide, pentoxide).
  • CO = carbon monoxide; CO₂ = carbon dioxide; N₂O₄ = dinitrogen tetroxide; SF₆ = sulfur hexafluoride.
  • No Roman numerals or charges here; H₂O/NH₃/CH₄ use common names.

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Practice General Chemistry I

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Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Name binary molecular compounds using Greek numerical prefixes.
  • Write formulas from molecular compound names, translating prefixes into subscripts.
  • State when the prefix "mono-" is used and when it is dropped.
  • Distinguish molecular nomenclature from ionic nomenclature (no charges, no Roman numerals).

Sources & references

  1. OpenStax, *Chemistry 2e*, Ch. 2.7, "Chemical Nomenclature."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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