Chemistry Nomenclature: Naming Ionic and Molecular Compounds
Chemistry nomenclature turns compound names and formulas into a clear decision process instead of random vocabulary. This unit builds from periodic table trends and sets up moles and reactions.
What you'll learn
6.1 Start Here: What Nomenclature Is and Why It Matters
Nomenclature is just the system chemists use for naming substances. Once the rules are clear, compound names stop feeling random.
You need both directions:
- 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.
Later units assume you can move both ways without guessing: name the formula you see, and write the formula for the name you are given.
If not, is a metal or NH4+ present? → ionic
If not, only nonmetals → molecular
Note on (aq): The symbol (aq) just means a compound is dissolved in water. Both acids and ionic compounds appear with (aq) — for example, NaCl(aq) and Ca(NO3)2(aq) are ionic, not acids. Use H written first in the formula — not (aq) — to spot an acid.
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 Writing Formulas from Names
Going name → formula is the harder direction because you construct the subscripts from scratch instead of reading them off a formula. The payoff is that the words in the name give you every clue you need — the name identifies the compound type, and the compound type tells you the formula-building rule.
- Name ends in "…acid" → acid rules apply
- Name contains a Roman numeral → ionic, variable-charge metal
- Name contains a polyatomic ion name (sulfate, nitrate, hydroxide…) → ionic
- Name has prefix words (di-, tri-, penta-…) → molecular
- Metal name + nonmetal root ending in –ide, nothing else → simple ionic
Once you identify the type, apply the matching formula-building rule:
Write each ion with its charge. Find the smallest whole-number ratio that makes the total charge exactly zero. Those ratios are the subscripts — drop any subscript of 1.
Example: potassium oxide → K+ and O2-. Need 2 K+ to cancel 1 O2- → K2O
The Roman numeral is the metal's charge — use it directly. Write the metal ion with that charge, write the anion with its charge, then balance to zero for the subscripts.
Example: copper(II) oxide → Cu2+ and O2-. Equal and opposite charges, 1:1 ratio → CuO
Look up the polyatomic ion's formula and charge in the 6.4 table. Balance it against the cation charge to get subscripts. If the polyatomic ion needs a subscript greater than 1, enclose the whole ion in parentheses before adding the subscript.
Example: calcium nitrate → Ca2+ and NO3-. Need 2 nitrate ions to cancel one calcium → Ca(NO3)2
Each prefix tells you the subscript directly. First element: use the prefix as the subscript; no prefix means 1. Second element: use its prefix as the subscript, then drop the "–ide" ending to get the element symbol.
Example: dinitrogen monoxide → di = 2 nitrogen, mono = 1 oxygen → N2O
Binary acid (starts with "hydro-"): strip "hydro-" and "-ic acid," match the remaining root to its element, then write H + that element. Use charge balance for the subscript — most are 1:1 (HCl, HBr, HI, HF) except hydrosulfuric acid where sulfur is 2- so two H are needed.
Oxyacid (no "hydro-"): –ic acid means the ion inside ends in –ate; –ous acid means the ion ends in –ite. Look up that polyatomic ion's formula and charge in the 6.4 table. Add enough H+ (each +1) to make the total charge zero.
Example: sulfuric acid → –ic acid, no hydro- → sulfate = SO42-. Need 2 H+ to balance -2 → H2SO4
Parentheses: when and why
- Use parentheses only around a polyatomic ion that appears more than once.
- Correct: Ca(NO3)2 — two nitrate groups, so they go in parentheses.
- Correct: NaNO3 — only one nitrate group, so no parentheses.
- Never put parentheses around a single-element ion like Cl or O.
| Name | Type signal | Formula |
|---|---|---|
| potassium bromide | metal + –ide, no Roman numeral → simple ionic | KBr |
| iron(III) chloride | Roman numeral III → Fe is 3+ | FeCl3 |
| calcium nitrate | nitrate is polyatomic → ionic | Ca(NO3)2 |
| dinitrogen monoxide | di- and mono- → molecular | N2O |
| hydroiodic acid | hydro-…-ic acid → binary acid | HI |
| nitric acid | –ic acid, no hydro- → nitrate inside → oxyacid | HNO3 |
Best way to lock in Unit 06
After the Unit 06 Practice page, use the broader practice hub for more naming reps and pair this unit with The Flashcard Method That Works if polyatomic ions, acids, and Roman numeral patterns still need cleaner recall.