Nomenclature
Start here if compound names still feel like random vocabulary. This unit turns naming into a decision process — building from periodic table trends and setting up moles and reactions.
What you'll learn
6.1 Start Here: What Nomenclature Is and Why It Matters
Nomenclature means the rules for naming things. In chemistry, nomenclature rules let chemists name millions of compounds in a clear, organized way.
When you see a chemical name, you should be able to figure out what atoms are in the compound. When you see a formula, you should be able to write the name. Both skills matter, because later units assume you can move both directions without guessing.
If not, metal or NH₄⁺ present? → ionic
If not, only nonmetals remain → molecular
6.2 Ionic Compounds: Metal First, Nonmetal Second
Many ionic compounds are made of a metal and a nonmetal. In these compounds, positive and negative ions attract each other.
Later, you will also see ionic compounds that contain polyatomic ions, such as ammonium or sulfate. Start with the core pattern: metal first, nonmetal second. Once this is automatic, 6.4 adds polyatomic ions and 6.3 adds the Roman numeral step for variable-charge metals.
Start here with the two-step pattern:
Write the name of the metal exactly as it appears on the periodic table. Do not change it.
Drop the end of the nonmetal's name and add –ide. For example: chlorine → chloride, oxygen → oxide, sulfur → sulfide, nitrogen → nitride, fluorine → fluoride, bromine → bromide, iodine → iodide.
Here are the most common simple ionic compounds. Notice that none of the names use prefixes — the subscripts in the formula handle the ratio automatically.
| Formula | Metal Ion | Nonmetal Ion | Name |
|---|---|---|---|
| NaCl | sodium | chloride | sodium chloride |
| K₂O | potassium | oxide | potassium oxide |
| MgBr₂ | magnesium | bromide | magnesium bromide |
| Al₂O₃ | aluminum | oxide | aluminum oxide |
| CaF₂ | calcium | fluoride | calcium fluoride |
| Li₃N | lithium | nitride | lithium nitride |
- Do not use prefixes like "di-" or "tri-" for ionic compounds.
- The subscripts in the formula handle the ratios automatically.
- Common mistake: seeing a subscript of 2 and trying to say "dichloride" in an ionic name. Do not do that.
6.3 Transition Metals: When You Need Roman Numerals
Some metals can form more than one type of ion. These are mainly transition metals like iron (Fe), copper (Cu), and lead (Pb).
When a metal can form more than one charge, you must add a Roman numeral in parentheses after the metal name. The Roman numeral shows the charge of the metal ion in that compound. The numeral comes from charge balance, not from the subscript by itself.
| Formula | Metal Charge | Name |
|---|---|---|
| FeCl₂ | Fe²⁺ | iron(II) chloride |
| FeCl₃ | Fe³⁺ | iron(III) chloride |
| CuO | Cu²⁺ | copper(II) oxide |
| Cu₂O | Cu⁺ | copper(I) oxide |
| FeO | Fe²⁺ | iron(II) oxide |
| Fe₂O₃ | Fe³⁺ | iron(III) oxide |
- Metals that always form one charge, like Na⁺, K⁺, Mg²⁺, Al³⁺, and Ca²⁺, do not need Roman numerals.
- Only use them when the metal can have more than one charge.
6.4 Polyatomic Ions: Keep the Ion Name
A polyatomic ion is a group of atoms bonded together that carries a charge. It acts as one unit in a formula. These have to be memorized. There is no shortcut for the table itself — but the naming move is simple: if the ion is polyatomic, keep its memorized name exactly as given.
Use this as your study reference. The ones you will see most often in this course are ammonium, hydroxide, nitrate, sulfate, carbonate, and phosphate — start there.
| Ion | Name | Charge |
|---|---|---|
| NH₄⁺ | ammonium | 1+ |
| OH⁻ | hydroxide | 1− |
| NO₃⁻ | nitrate | 1− |
| NO₂⁻ | nitrite | 1− |
| SO₄²⁻ | sulfate | 2− |
| SO₃²⁻ | sulfite | 2− |
| CO₃²⁻ | carbonate | 2− |
| HCO₃⁻ | hydrogen carbonate | 1− |
| PO₄³⁻ | phosphate | 3− |
| ClO₄⁻ | perchlorate | 1− |
| ClO₃⁻ | chlorate | 1− |
| ClO₂⁻ | chlorite | 1− |
| ClO⁻ | hypochlorite | 1− |
| MnO₄⁻ | permanganate | 1− |
| CrO₄²⁻ | chromate | 2− |
| Cr₂O₇²⁻ | dichromate | 2− |
| C₂H₃O₂⁻ | acetate | 1− |
| CN⁻ | cyanide | 1− |
When naming a compound with a polyatomic ion, use the name of the ion directly. Do not change it to "-ide."
Examples: Ca(OH)₂ = calcium hydroxide · NH₄NO₃ = ammonium nitrate · Na₂SO₄ = sodium sulfate
6.5 Molecular Compounds: Nonmetal + Nonmetal Means Prefixes
A molecular compound is made of nonmetals bonded together. Nonmetals can bond in many different ratios, so we use prefixes to show exactly how many atoms of each element are in the molecule.
These ten prefixes are the only ones you need for molecular compounds. The second element always gets a prefix; the first element skips "mono" when there is only one.
| Number | Prefix | Number | Prefix |
|---|---|---|---|
| 1 | mono- | 6 | hexa- |
| 2 | di- | 7 | hepta- |
| 3 | tri- | 8 | octa- |
| 4 | tetra- | 9 | nona- |
| 5 | penta- | 10 | deca- |
If there is only 1 atom of the first element, skip "mono." If there are 2 or more, use the prefix that matches the subscript.
Always use a prefix for the second element. Change the ending to –ide. If the prefix ends in a or o and the element name starts with a vowel, drop that vowel from the prefix (e.g., tetra- + oxide = tetroxide, not tetraoxide).
| Formula | Name |
|---|---|
| CO₂ | carbon dioxide |
| CO | carbon monoxide |
| N₂O₄ | dinitrogen tetroxide |
| SF₆ | sulfur hexafluoride |
| PCl₃ | phosphorus trichloride |
| PCl₅ | phosphorus pentachloride |
| N₂O | dinitrogen monoxide |
| NO₂ | nitrogen dioxide |
| SO₃ | sulfur trioxide |
| P₄O₁₀ | tetraphosphorus decoxide |
The biggest mistake
- Using prefixes for ionic compounds.
- Only use prefixes for molecular compounds (nonmetal + nonmetal).
- If a polyatomic ion is present in an ionic compound, keep its memorized name instead of changing it to -ide.
- The subscripts in ionic compounds come from the ion charges. They are not a naming choice.
6.6 Binary Acids: No Oxygen, Use Hydro-
In this unit, acid names are used when the compound is acting as an acid in water.
A binary acid contains only two elements: hydrogen and one other nonmetal, with no oxygen. Start here with the fastest check: no oxygen means binary acid.
All binary acids start with "hydro-"
For chlorine → chlor, bromine → brom, iodine → iod, sulfur → sulfur, fluorine → fluor
| Formula | Name |
|---|---|
| HCl(aq) | hydrochloric acid |
| HBr(aq) | hydrobromic acid |
| HI(aq) | hydroiodic acid |
| HF(aq) | hydrofluoric acid |
| H₂S(aq) | hydrosulfuric acid |
6.7 Oxyacids: Oxygen Present, Change the Ion Ending
An oxyacid contains hydrogen, oxygen, and one other element. You name oxyacids based on the polyatomic ion inside the acid. Notice: this is where the -ate and -ite endings from 6.4 connect to acid names. The rule is a direct translation of the ion ending.
If the ion ends in –ite → the acid ends in –ous acid
Each row shows the polyatomic ion inside the acid and how its ending changes to produce the acid name.
| Ion Inside | Ion Name | Acid | Acid Name |
|---|---|---|---|
| CO₃²⁻ | carbonate (–ate) | H₂CO₃ | carbonic acid (–ic) |
| SO₄²⁻ | sulfate (–ate) | H₂SO₄ | sulfuric acid (–ic) |
| SO₃²⁻ | sulfite (–ite) | H₂SO₃ | sulfurous acid (–ous) |
| NO₃⁻ | nitrate (–ate) | HNO₃ | nitric acid (–ic) |
| NO₂⁻ | nitrite (–ite) | HNO₂ | nitrous acid (–ous) |
| PO₄³⁻ | phosphate (–ate) | H₃PO₄ | phosphoric acid (–ic) |
| ClO₄⁻ | perchlorate (–ate) | HClO₄ | perchloric acid (–ic) |
| ClO₃⁻ | chlorate (–ate) | HClO₃ | chloric acid (–ic) |
| ClO₂⁻ | chlorite (–ite) | HClO₂ | chlorous acid (–ous) |
| ClO⁻ | hypochlorite (–ite) | HClO | hypochlorous acid (–ous) |
- Drop the "hydro-" prefix for oxyacids.
- Only binary acids with no oxygen use "hydro-".
- Oxyacids use the ion name with the changed ending.
6.8 How to Tell Compound Types Apart
You now have all four compound types covered. This section is the synthesis: one decision path that connects 6.2 through 6.7. Use the shortest possible decision path. If this part gets automatic, the rest of nomenclature gets much easier.
If yes, use acid rules. No oxygen means binary acid. Oxygen means oxyacid.
If yes, it is ionic. Fixed-charge metals need no Roman numeral. Variable-charge metals do. Polyatomic ions keep their ion names.
That means molecular. Use prefixes, skip mono- on the first element, and always use a prefix on the second element.
| Formula clue | Type | Naming move |
|---|---|---|
| HNO₃(aq) | Acid | Contains oxygen, so use the oxyacid ending rule. |
| CaCO₃ | Ionic with polyatomic ion | Name the metal, then keep the ion name carbonate. |
| FeCl₃ | Ionic with variable-charge metal | Find the iron charge first, then add a Roman numeral. |
| CO₂ | Molecular | Use prefixes because both elements are nonmetals. |