Naming Ionic vs Molecular Compounds: Which Rules to Use and When

Why is CO₂ called carbon dioxide while MgCl₂ is called magnesium chloride and not “magnesium dichloride”? Both formulas have a subscript 2. Both names end in -ide. Yet one name counts the atoms and the other does not.

The reason is that chemists use two different naming systems, one for ionic compounds and one for molecular (covalent) compounds, and each system is built around what the formula actually tells you. Mixing them up is one of the most common errors in early chemistry, and it is easy to avoid once you know which system to reach for. This guide sets the two side by side, shows how to decide which one applies, and lists the exceptions worth memorising.

Step zero: decide which kind of compound you have

Before you name anything, classify it. A quick test works for almost every compound you will meet in school chemistry:

• Does the formula contain a metal, or the ammonium ion (NH₄⁺)? Treat it as ionic.
• Is it made only of non-metals (and not ammonium)? Treat it as molecular.

So NaCl, Fe₂O₃, CuSO₄ and NH₄NO₃ are named as ionic compounds. CO, N₂O₄, PCl₅ and SF₆ are named as molecular compounds. Ammonium compounds are the one case where there is no metal in sight but the ionic system still applies, because NH₄⁺ behaves as a positive ion just like Na⁺.

The deeper reason for the split is covered in ionic vs covalent bonds, but for naming you only need the rule above.

Side-by-side comparison

Feature	Ionic compounds	Molecular compounds
Made of	A metal (or NH₄⁺) and a non-metal or polyatomic ion	Non-metals only
What the formula shows	The simplest ratio of ions in a lattice	The actual number of atoms in one molecule
First part of the name	The metal’s name, unchanged	The first element’s name, with a prefix if more than one atom
Second part of the name	Non-metal root + -ide, or the polyatomic ion’s name	Second element’s root + -ide, always with a prefix
Greek prefixes (di-, tri-…)	Never	Always on the second element; on the first if there is more than one atom
Roman numerals	Yes, for metals that form more than one ion, e.g. iron(II), iron(III)	No
How you get the ratio from the name	Balance the ion charges	Read it straight from the prefixes
Examples	NaCl sodium chloride; MgCl₂ magnesium chloride; FeCl₃ iron(III) chloride; Na₂SO₄ sodium sulfate	CO carbon monoxide; CO₂ carbon dioxide; N₂O₄ dinitrogen tetroxide; SF₆ sulfur hexafluoride

Why ionic names don’t count atoms

In an ionic compound the ratio of ions is fixed by their charges. Magnesium forms only Mg²⁺ and chlorine forms only Cl⁻, so there is exactly one way to make a neutral compound: one Mg²⁺ for every two Cl⁻. Saying “magnesium dichloride” would add information that nobody needs. The name “magnesium chloride” already tells you everything, because the charges do the counting for you.

The rules are short:

1. Name the positive ion first, using the metal’s name unchanged: sodium, calcium, aluminium.
2. Name the negative ion second. For a single non-metal, take the root and add -ide: chloride, oxide, sulfide, nitride, fluoride. For a polyatomic ion, use its own name: sulfate, nitrate, carbonate, hydroxide, phosphate.
3. If the metal can form more than one ion (most transition metals, plus tin and lead), show its charge with a Roman numeral in brackets straight after the name: copper(I) oxide is Cu₂O and copper(II) oxide is CuO.

The full routine, with more polyatomic ions and hydrates, is in how to name ionic compounds. A reference list of the ions themselves is in polyatomic ions list.

Why molecular names must count atoms

Non-metals can combine in several ratios, and there are no ion charges to fix the ratio for you. Carbon and oxygen give both CO and CO₂. Nitrogen and oxygen give NO, NO₂, N₂O, N₂O₄ and N₂O₅, among others. A name like “nitrogen oxide” would be ambiguous, so molecular names spell out the number of each atom with prefixes.

Number	Prefix	Number	Prefix
1	mono-	6	hexa-
2	di-	7	hepta-
3	tri-	8	octa-
4	tetra-	9	nona-
5	penta-	10	deca-

The rules:

1. Name the first element in full. Add a prefix only if there are two or more atoms of it. Mono- is dropped on the first element, so CO is carbon monoxide, not “monocarbon monoxide”.
2. Name the second element with its -ide ending and always add a prefix, including mono- when there is one atom.
3. When a prefix ending in a or o meets “oxide”, school textbooks usually drop the vowel: monoxide, tetroxide, pentoxide. Current IUPAC recommendations keep it (tetraoxide) except in monoxide, so you will see both spellings, and both are accepted. The i of di- and tri- is never dropped: triiodide.
4. The element written first is normally the one further to the left (or lower) in the periodic table. Oxygen goes last in most names, except with fluorine: OF₂ is oxygen difluoride.

Worked examples: N₂O₄ is dinitrogen tetroxide; PCl₅ is phosphorus pentachloride; SO₃ is sulfur trioxide; CCl₄ is carbon tetrachloride; N₂O is dinitrogen monoxide; P₄O₁₀ is tetraphosphorus decoxide (or decaoxide). The dedicated guide to naming covalent compounds has more practice.

Common names that break the pattern

Some molecular compounds were named long before the systematic rules existed, and their traditional names are the ones everybody uses. You are expected to know these:

• H₂O is water (never “dihydrogen monoxide”, except as a joke).
• NH₃ is ammonia.
• CH₄ is methane, and other hydrocarbons follow organic naming rules instead of prefixes.
• N₂H₄ is hydrazine.

Hydrogen compounds of the halogens and group 16 are also usually named without prefixes: HCl is hydrogen chloride, H₂S is hydrogen sulfide. Dissolved in water, the halogen compounds switch to acid names (hydrochloric acid), which follow their own rules.

Borderline cases

Real bonding is a spectrum, and a few compounds sit awkwardly between the two naming systems. Aluminium chloride is largely covalent in character, yet it contains a metal and is named the ionic way: aluminium chloride. Tin(IV) chloride, SnCl₄, is a liquid made of molecules, but it too is usually named with a Roman numeral. In school chemistry, the metal test wins: if there is a metal, use ionic naming. You may occasionally see older or alternative names such as “tin tetrachloride”; recognise them, but don’t write them in an exam that asks for the systematic name.

When to use which system: a quick decision path

1. Metal or NH₄⁺ present? → ionic rules.
   • Does the metal form more than one ion? → add a Roman numeral. Work out the charge from the anion; the oxidation number calculator can check your answer.
   • Is there a polyatomic ion? → use its name, don’t change it to -ide (except hydroxide and cyanide, which already end in -ide).
2. Non-metals only? → molecular rules.
   • Is it water, ammonia, methane or another traditional name? → use that.
   • Otherwise → prefixes, dropping mono- on the first element only.

Common mistakes

• Prefixes on ionic compounds. “Calcium dichloride” and “disodium oxide” are wrong. Write calcium chloride and sodium oxide.
• Forgetting prefixes on molecular compounds. “Carbon oxide” does not tell you whether you mean CO or CO₂.
• Reading the Roman numeral as a count of atoms. Iron(III) oxide is Fe₂O₃. The III is the charge on each iron ion, not the number of oxygen atoms.
• Putting mono- on the first element. CO is carbon monoxide, not monocarbon monoxide.
• Leaving the element name unchanged in the second position. Sodium chlorine, carbon dioxygen: the second element always takes -ide.
• Confusing -ide, -ite and -ate. Sulfide is S²⁻; sulfite is SO₃²⁻; sulfate is SO₄²⁻. They are different ions.
• Adding a Roman numeral to a metal with only one ion. Sodium(I) chloride and zinc(II) oxide are not wrong in meaning, but they are not the standard names; write sodium chloride and zinc oxide.

For a set of mixed questions that drill the decision between the two systems, try naming compounds practice.

Key takeaways

• Classify first: a metal or ammonium means ionic naming; non-metals only means molecular naming.
• Ionic names never use Greek prefixes, because ion charges already fix the ratio. Roman numerals show the charge of metals with more than one possible ion.
• Molecular names always use prefixes on the second element, and on the first element only when there are two or more atoms.
• Water, ammonia and methane keep their traditional names.
• Most naming errors come from using the right rules on the wrong type of compound.