Naming chemical formulas follows a set of rules that depend on what type of compound you’re looking at. The system is governed by IUPAC (the International Union of Pure and Applied Chemistry), which sets the global standard so that every chemist, manufacturer, and regulator means the same thing when they write a name. Once you learn to identify whether a compound is ionic, molecular, or an acid, the naming rules become straightforward patterns you can apply to nearly any formula.
Identify the Compound Type First
Before you can name anything, you need to figure out what kind of compound you’re dealing with. This single step determines which set of rules to follow. There are four main categories you’ll encounter in general chemistry:
- Binary ionic compounds: a metal bonded to a nonmetal (like NaCl)
- Ionic compounds with polyatomic ions: groups of atoms that carry a charge (like CaCO₃)
- Molecular (covalent) compounds: two nonmetals bonded together (like CO₂)
- Acids: compounds that start with hydrogen and dissolve in water to release H⁺ (like HCl)
A quick way to sort them: if the formula starts with a metal, it’s ionic. If it starts with a nonmetal bonded to another nonmetal (and isn’t an acid), it’s molecular. If it starts with H and isn’t water, it’s likely an acid.
Naming Binary Ionic Compounds
Binary ionic compounds contain exactly two elements: a metal and a nonmetal. The rules are simple. Write the metal’s name first, exactly as it appears on the periodic table. Then write the nonmetal’s name, but change the ending to “-ide.” That’s it.
NaCl becomes sodium chloride. MgO becomes magnesium oxide. CaBr₂ becomes calcium bromide. Notice that you never use Greek prefixes to indicate how many atoms are present. You don’t call Na₂O “disodium oxide.” The name is just sodium oxide. The ratio of atoms is implied by the charges of the ions.
Transition Metals and Roman Numerals
Metals like iron, copper, and tin can form ions with different charges. Iron, for example, can be Fe²⁺ or Fe³⁺. To tell the reader which one you mean, you add the charge as a Roman numeral in parentheses right after the metal’s name. FeCl₂ is iron(II) chloride because each chloride ion has a 1- charge, meaning two of them balance a 2+ iron. FeCl₃ is iron(III) chloride.
You need Roman numerals for most transition metals and a handful of other metals with variable charges (like lead and tin). Metals that only form one charge, such as sodium (always 1+), calcium (always 2+), and aluminum (always 3+), never get Roman numerals because there’s no ambiguity.
Naming Compounds With Polyatomic Ions
Polyatomic ions are clusters of atoms that act as a single charged unit. You’ll need to memorize the most common ones, because their names don’t follow a pattern you can derive from scratch. Here are some of the ones you’ll see most often:
- Hydroxide: OH⁻
- Nitrate: NO₃⁻
- Nitrite: NO₂⁻
- Sulfate: SO₄²⁻
- Sulfite: SO₃²⁻
- Ammonium: NH₄⁺
- Phosphate: PO₄³⁻
The naming process is identical to binary ionic compounds: metal name first, then the polyatomic ion’s name. CaCO₃ is calcium carbonate. NaOH is sodium hydroxide. If the metal has a variable charge, use Roman numerals the same way: Fe(NO₃)₃ is iron(III) nitrate.
One useful pattern to notice: ions ending in “-ate” have more oxygen atoms than their “-ite” counterparts. Sulfate (SO₄²⁻) has one more oxygen than sulfite (SO₃²⁻). Nitrate (NO₃⁻) has one more oxygen than nitrite (NO₂⁻). This pattern holds across many pairs and makes memorization easier.
Naming Molecular (Covalent) Compounds
When two nonmetals bond, the rules change. Instead of relying on ion charges, you use Greek prefixes to state exactly how many atoms of each element are in the formula. The prefixes you need are:
- 1 = mono
- 2 = di
- 3 = tri
- 4 = tetra
- 5 = penta
- 6 = hexa
- 7 = hepta
- 8 = octa
- 9 = nona
- 10 = deca
Name the first element with its prefix, then name the second element with its prefix and change the ending to “-ide.” CO₂ becomes carbon dioxide. N₂O₃ becomes dinitrogen trioxide. N₂S becomes dinitrogen monosulfide.
There are a few readability rules that clean up awkward combinations. First, drop the “mono” prefix on the first element. You say carbon dioxide, not monocarbon dioxide. Second, if the first element is hydrogen, drop all prefixes entirely: HCl is hydrogen chloride, not monohydrogen monochloride. Third, when a prefix ending in “o” or “a” meets an element name starting with a vowel, drop that last vowel from the prefix. That’s why CO is carbon monoxide (not monooxide) and N₂O₄ is dinitrogen tetroxide (not tetraoxide).
Naming Acids
Acids follow their own naming system, and it splits into two categories depending on whether the acid contains oxygen.
Binary Acids (No Oxygen)
A binary acid is hydrogen plus one other nonmetal, with no oxygen involved. The pattern is: start with “hydro-,” add the root of the nonmetal’s name, and end with “-ic acid.” HCl is hydrochloric acid. HBr is hydrobromic acid. HF is hydrofluoric acid. The “hydro-” prefix is your signal that there’s no oxygen in the acid.
Oxyacids (Contain Oxygen)
When an acid contains hydrogen, oxygen, and a third element, it’s called an oxyacid. These are named based on the polyatomic ion they come from, and the ending of that ion tells you what to do:
- If the ion ends in “-ate”: change it to “-ic acid.” Sulfate → sulfuric acid (H₂SO₄). Nitrate → nitric acid (HNO₃).
- If the ion ends in “-ite”: change it to “-ous acid.” Sulfite → sulfurous acid (H₂SO₃). Nitrite → nitrous acid (HNO₂).
Notice there’s no “hydro-” prefix on oxyacids. That prefix is reserved exclusively for binary acids. If you see “hydro-” in an acid name, you know it has no oxygen. If you see “-ic acid” or “-ous acid” without “hydro-,” you know oxygen is present.
Naming Hydrates
Some ionic compounds crystallize with water molecules trapped in their structure. These are called hydrates, and naming them just adds one step to the normal ionic naming process. Write the full ionic compound name first, then add a Greek prefix plus “hydrate” to indicate how many water molecules are present per formula unit.
CuSO₄·5H₂O is copper(II) sulfate pentahydrate. Ba(OH)₂·8H₂O is barium hydroxide octahydrate. The dot in the formula separates the ionic compound from the water molecules, and the prefix tells you the count.
Organic Compounds: The Basics
Organic chemistry has its own elaborate naming system, but the foundation starts with how many carbon atoms are in the longest chain. Each chain length gets a root name:
- 1 carbon: meth-
- 2 carbons: eth-
- 3 carbons: prop-
- 4 carbons: but-
- 5 carbons: pent-
- 6 carbons: hex-
- 7 carbons: hept-
- 8 carbons: oct-
- 9 carbons: non-
- 10 carbons: dec-
For simple hydrocarbons with only single bonds (alkanes), you add “-ane” to the root. One carbon with hydrogen is methane. Two carbons is ethane. Eight is octane. The detailed rules of IUPAC organic nomenclature, published in what’s known as the Blue Book (most recently revised in December 2023), cover branching, functional groups, and naming priority. But these root names are the starting point for everything in organic chemistry.
Common Names That Override the Rules
Some compounds have been known for so long that their traditional names have stuck, and even chemists rarely use the systematic version. Water is water, not dihydrogen monoxide. Ammonia is ammonia, not nitrogen trihydride. Methane keeps its organic name even in general chemistry contexts. You’ll pick these up naturally as you encounter them, and your course or textbook will generally flag when a common name is preferred over the systematic one.