Naming Chemical Compounds
How to look at a chemical formula and figure out what to call it!
What's in a Name?
So, you've learned how to read chemical formulas. You know that NaCl contains sodium and chlorine, and that KOH contains potassium, oxygen, and hydrogen.
But here's the next challenge: How do you actually SAY the name of a chemical compound?
For example:
NaCl = sodium chloride
KOH = potassium hydroxide
CaCO₃ = calcium carbonate
CO₂ = carbon dioxide
FeCl₃ = iron(III) chloride
Why does chlorine become chloride? Why is CO₂ carbon dioxide instead of carbon oxide? And what's with the Roman numeral in iron(III) chloride?
Is chemistry just making up words?
Well... not entirely.
Chemical compounds follow naming rules. Once you recognize a few common prefixes, suffixes, and groups of atoms, you can figure out the names of a LOT of compounds.
It's kind of like learning the vocabulary of a new language. Except this language has molecules, charges, and the occasional Roman numeral.
Let's decode it.
1. The First Rule: What Kind of Compound Is It?
Before naming a compound, look at the elements it contains.
For now, we'll focus on two major categories: ionic compounds and molecular (covalent) compounds.
Ionic Compounds: Name the Parts
Ionic compounds usually contain a metal and a nonmetal, or a metal and a group of atoms called a polyatomic ion.
The basic rule is:
Name the positive ion first. Name the negative ion second.
For example:
NaCl = sodium chloride
Na = sodium
Cl = chlorine, which becomes chloride when it is a negative ion.
KOH = potassium hydroxide
K = potassium
OH = hydroxide, a group of atoms that behaves as one charged unit.
Notice that we don't say "potassium oxygen hydrogen." The OH group already has its own name!
Another example:
CaCO₃ = calcium carbonate
Ca = calcium
CO₃ = carbonate
Some ionic compounds also contain polyatomic positive ions, such as ammonium (NH₄⁺). We'll include those in the reference list.
Molecular (Covalent) Compounds: Count the Atoms
Molecular compounds generally contain nonmetals bonded to other nonmetals.
For simple compounds containing two different nonmetals, we often use prefixes to tell us how many atoms of each element are present.
For example:
CO₂ = carbon dioxide
C = carbon
O₂ = two oxygen atoms
di- = two
-oxide = the naming form of oxygen
Put them together: carbon dioxide.
N₂O₄ = dinitrogen tetroxide
di- = two nitrogen atoms
tetra- = four oxygen atoms (shortened to tetr- before oxide)
The prefixes basically tell you the atom count. Chemistry is giving you the answers in the name!
Important: Ionic compounds generally do NOT use these counting prefixes. CaCl₂ is calcium chloride, NOT calcium dichloride.
2. Suffixes: The Endings That Tell You Something
A suffix is a word ending that provides information about a chemical substance.
Three of the most useful suffixes in introductory chemistry are:
-ide: Usually a Simple Negative Ion
When a single nonmetal atom forms a negative ion, its name usually ends in -ide.
Examples:
| Element | Negative ion name |
|---|---|
| Chlorine (Cl) | Chloride (Cl⁻) |
| Fluorine (F) | Fluoride (F⁻) |
| Bromine (Br) | Bromide (Br⁻) |
| Oxygen (O) | Oxide (O²⁻) |
| Sulfur (S) | Sulfide (S²⁻) |
| Nitrogen (N) | Nitride (N³⁻) |
| Phosphorus (P) | Phosphide (P³⁻) |
So:
NaCl = sodium chloride
MgO = magnesium oxide
CaS = calcium sulfide
AlN = aluminum nitride
The -ide ending is also used when naming the second element in many binary molecular compounds, such as carbon dioxide.
Heads up! Not every ion ending in -ide is made of just one atom. Hydroxide (OH⁻) and cyanide (CN⁻), for example, are polyatomic ions.
-ate: A Common Polyatomic Ion Ending
Many groups of atoms containing oxygen have names ending in -ate.
Examples:
NO₃⁻ = nitrate
SO₄²⁻ = sulfate
CO₃²⁻ = carbonate
PO₄³⁻ = phosphate
When you see these groups in a formula, use their names as complete units.
Examples:
NaNO₃ = sodium nitrate
K₂SO₄ = potassium sulfate
CaCO₃ = calcium carbonate
AlPO₄ = aluminum phosphate
-ite: A Related Ion With Less Oxygen
Here's a useful pattern.
When two related oxygen-containing ions have the same central element, the -ite ion generally has fewer oxygen atoms than the -ate ion.
| -ate ion | -ite ion |
|---|---|
| NO₃⁻ = nitrate | NO₂⁻ = nitrite |
| SO₄²⁻ = sulfate | SO₃²⁻ = sulfite |
| PO₄³⁻ = phosphate | PO₃³⁻ = phosphite |
| ClO₃⁻ = chlorate | ClO₂⁻ = chlorite |
Notice that each pair shown has the same charge. What changes is the number of oxygen atoms.
Memory trick: -ate has more oxygen; -ite has less oxygen (when comparing members of the same ion family).
Some families take this even further:
ClO⁻ = hypochlorite
ClO₂⁻ = chlorite
ClO₃⁻ = chlorate
ClO₄⁻ = perchlorate
In this family, hypo- means one fewer oxygen than chlorite, and per- means one more oxygen than chlorate.
3. Prefixes: How Many Atoms?
When naming simple molecular compounds containing two different nonmetals, Greek prefixes tell us how many atoms of each element are present.
| Prefix | Number |
|---|---|
| mono- | 1 |
| di- | 2 |
| tri- | 3 |
| tetra- | 4 |
| penta- | 5 |
| hexa- | 6 |
| hepta- | 7 |
| octa- | 8 |
| nona- | 9 |
| deca- | 10 |
Examples:
CO = carbon monoxide
CO₂ = carbon dioxide
SO₂ = sulfur dioxide
SO₃ = sulfur trioxide
N₂O = dinitrogen monoxide
N₂O₄ = dinitrogen tetroxide
PCl₃ = phosphorus trichloride
PCl₅ = phosphorus pentachloride
SF₆ = sulfur hexafluoride
A couple of important details:
We generally leave mono- off the FIRST element. CO is carbon monoxide, not monocarbon monoxide.
The SECOND element gets a counting prefix, including mono- when appropriate.
The second element's name ends in -ide.
Sometimes vowels are dropped to make pronunciation easier. For example, mono + oxide becomes monoxide, and tetra + oxide becomes tetroxide.
A Quick Warning
Not all compounds follow the simple prefix system in everyday use.
For example:
H₂O = water
NH₃ = ammonia
CH₄ = methane
H₂O₂ = hydrogen peroxide
These compounds have established names that students should recognize.
4. Roman Numerals: When Metals Have More Than One Charge
Some metals, especially transition metals, can form ions with different positive charges.
That creates a naming problem.
Consider iron:
Fe²⁺ = iron(II)
Fe³⁺ = iron(III)
The Roman numeral tells you the charge of the metal ion, NOT the number of metal atoms.
For example:
FeCl₂ = iron(II) chloride
Each chloride ion has a charge of 1−. Two chloride ions have a total charge of 2−, so the iron must have a charge of 2+.
FeCl₃ = iron(III) chloride
Three chloride ions have a total charge of 3−, so the iron must have a charge of 3+.
Other examples:
CuCl = copper(I) chloride
CuCl₂ = copper(II) chloride
Fe₂O₃ = iron(III) oxide
CuSO₄ = copper(II) sulfate
Metals that normally form only one common ion charge, such as sodium (Na⁺), calcium (Ca²⁺), and aluminum (Al³⁺), do not need Roman numerals in their compound names.
5. Polyatomic Ions: Groups That Stick Together
A polyatomic ion is a group of two or more bonded atoms that carries an overall electric charge.
Think of the atoms as a team. Instead of naming every player individually, you use the team's name.
For example:
OH⁻ = hydroxide
So:
NaOH = sodium hydroxide
KOH = potassium hydroxide
Ca(OH)₂ = calcium hydroxide
Notice the parentheses in Ca(OH)₂.
The subscript 2 tells us that there are TWO complete hydroxide groups.
Another example:
SO₄²⁻ = sulfate
So:
Na₂SO₄ = sodium sulfate
MgSO₄ = magnesium sulfate
Al₂(SO₄)₃ = aluminum sulfate
The names do not change just because there are multiple ions in a formula.
Remember: Subscripts tell you how many atoms or groups are present. Superscripts on ions tell you their electrical charges.
6. Special Case: Acids
Some common compounds have special names when dissolved in water.
NOTE: In most cases, acids will begin with hydrogen in the molecular notation.
For example:
HCl(g) = hydrogen chloride gas
HCl(aq) = hydrochloric acid
The symbol (aq) means aqueous, or dissolved in water.
For many common acids containing oxygen, the following naming patterns are useful:
An ion ending in -ate becomes an acid ending in -ic acid.
An ion ending in -ite becomes an acid ending in -ous acid.
Examples:
| Related ion | Acid formula | Acid name |
|---|---|---|
| Nitrate (NO₃⁻) | HNO₃(aq) | Nitric acid |
| Nitrite (NO₂⁻) | HNO₂(aq) | Nitrous acid |
| Sulfate (SO₄²⁻) | H₂SO₄(aq) | Sulfuric acid |
| Sulfite (SO₃²⁻) | H₂SO₃(aq) | Sulfurous acid |
| Phosphate (PO₄³⁻) | H₃PO₄(aq) | Phosphoric acid |
Binary acids such as HCl(aq) follow a different pattern, often using hydro- and -ic acid.
For now, recognizing the names of the most common acids is more important than memorizing every acid-naming rule.
Summary: The Naming Cheat Codes
When you're trying to name a compound, follow these ideas:
Identify the elements or ion groups. Know what Na, Cl, OH, SO₄, and other symbols represent.
Look for a metal or ammonium. These often indicate an ionic compound. Name the positive ion first and the negative ion second.
Watch for -ide. A single nonmetal negative ion usually changes its name to end in -ide.
Recognize -ate and -ite. These are common polyatomic ion endings. For related oxygen-containing ions, -ate generally has more oxygen than -ite.
Use prefixes with molecular compounds. Mono-, di-, tri-, and the others tell you how many atoms are present.
Check for Roman numerals. Some metals have more than one possible charge, and the Roman numeral identifies which one is present.
Learn common exceptions. Water, ammonia, methane, and many acids have established names that do not follow the simplest naming patterns.
Pay attention to charges. Ionic compounds must have a total electrical charge of zero.
The big idea: You don't have to memorize the names of every compound in existence. Learn the common elements, common ions, and naming patterns, and you can decode a LOT of chemical formulas.
Chemical Naming Reference Lists
The following lists contain common elements, ions, prefixes, and compounds students should recognize in a standard high school chemistry course.
A. Common Positive Ions (Cations)
Positive ions are usually named after the element they come from.
| Symbol | How to say it | Charge |
|---|---|---|
| H⁺ | Hydrogen ion | 1+ |
| Li⁺ | Lithium | 1+ |
| Na⁺ | Sodium | 1+ |
| K⁺ | Potassium | 1+ |
| Ag⁺ | Silver | 1+ |
| Mg²⁺ | Magnesium | 2+ |
| Ca²⁺ | Calcium | 2+ |
| Ba²⁺ | Barium | 2+ |
| Zn²⁺ | Zinc | 2+ |
| Al³⁺ | Aluminum | 3+ |
| Fe²⁺ | Iron(II) | 2+ |
| Fe³⁺ | Iron(III) | 3+ |
| Cu⁺ | Copper(I) | 1+ |
| Cu²⁺ | Copper(II) | 2+ |
| Sn²⁺ | Tin(II) | 2+ |
| Sn⁴⁺ | Tin(IV) | 4+ |
| Pb²⁺ | Lead(II) | 2+ |
| Pb⁴⁺ | Lead(IV) | 4+ |
| NH₄⁺ | Ammonium | 1+ |
NH₄⁺ is a polyatomic ion, but it belongs in the positive-ion list because it carries a positive charge.
B. Common Single-Atom Negative Ions (Anions)
| Symbol | Element name | Ion name | Charge |
|---|---|---|---|
| F⁻ | Fluorine | Fluoride | 1− |
| Cl⁻ | Chlorine | Chloride | 1− |
| Br⁻ | Bromine | Bromide | 1− |
| I⁻ | Iodine | Iodide | 1− |
| O²⁻ | Oxygen | Oxide | 2− |
| S²⁻ | Sulfur | Sulfide | 2− |
| N³⁻ | Nitrogen | Nitride | 3− |
| P³⁻ | Phosphorus | Phosphide | 3− |
| H⁻ | Hydrogen | Hydride | 1− |
C. Common Polyatomic Ions
These are especially important to recognize because they occur in many chemical reactions.
| Formula | How to say it | Charge |
|---|---|---|
| NH₄⁺ | Ammonium | 1+ |
| H₃O⁺ | Hydronium | 1+ |
| OH⁻ | Hydroxide | 1− |
| NO₃⁻ | Nitrate | 1− |
| NO₂⁻ | Nitrite | 1− |
| SO₄²⁻ | Sulfate | 2− |
| SO₃²⁻ | Sulfite | 2− |
| HSO₄⁻ | Hydrogen sulfate | 1− |
| CO₃²⁻ | Carbonate | 2− |
| HCO₃⁻ | Hydrogen carbonate (bicarbonate) | 1− |
| PO₄³⁻ | Phosphate | 3− |
| PO₃³⁻ | Phosphite | 3− |
| C₂H₃O₂⁻ | Acetate | 1− |
| CN⁻ | Cyanide | 1− |
| ClO⁻ | Hypochlorite | 1− |
| ClO₂⁻ | Chlorite | 1− |
| ClO₃⁻ | Chlorate | 1− |
| ClO₄⁻ | Perchlorate | 1− |
| MnO₄⁻ | Permanganate | 1− |
| CrO₄²⁻ | Chromate | 2− |
| Cr₂O₇²⁻ | Dichromate | 2− |
| O₂²⁻ | Peroxide | 2− |
D. Other Common Element Symbols
These element names are especially useful when reading molecular formulas.
| Symbol | Element name |
|---|---|
| H | Hydrogen |
| B | Boron |
| C | Carbon |
| N | Nitrogen |
| O | Oxygen |
| F | Fluorine |
| Si | Silicon |
| P | Phosphorus |
| S | Sulfur |
| Cl | Chlorine |
| Br | Bromine |
| I | Iodine |
Remember that uppercase and lowercase letters matter. Co is cobalt, while CO contains carbon and oxygen.
E. Common Chemical Compounds to Recognize
Ionic Compounds
| Chemical formula | Compound name |
|---|---|
| NaCl | Sodium chloride |
| KCl | Potassium chloride |
| CaCl₂ | Calcium chloride |
| MgCl₂ | Magnesium chloride |
| MgO | Magnesium oxide |
| CaO | Calcium oxide |
| Al₂O₃ | Aluminum oxide |
| NaOH | Sodium hydroxide |
| KOH | Potassium hydroxide |
| Ca(OH)₂ | Calcium hydroxide |
| Mg(OH)₂ | Magnesium hydroxide |
| NaNO₃ | Sodium nitrate |
| KNO₃ | Potassium nitrate |
| AgNO₃ | Silver nitrate |
| Na₂SO₄ | Sodium sulfate |
| MgSO₄ | Magnesium sulfate |
| CuSO₄ | Copper(II) sulfate |
| Al₂(SO₄)₃ | Aluminum sulfate |
| CaCO₃ | Calcium carbonate |
| Na₂CO₃ | Sodium carbonate |
| NaHCO₃ | Sodium hydrogen carbonate (baking soda) |
| Na₃PO₄ | Sodium phosphate |
| Ca₃(PO₄)₂ | Calcium phosphate |
| NH₄Cl | Ammonium chloride |
| NH₄NO₃ | Ammonium nitrate |
| (NH₄)₂SO₄ | Ammonium sulfate |
| NaC₂H₃O₂ | Sodium acetate |
| NaClO | Sodium hypochlorite |
| KClO₃ | Potassium chlorate |
| FeCl₂ | Iron(II) chloride |
| FeCl₃ | Iron(III) chloride |
| Fe₂O₃ | Iron(III) oxide |
| Cu₂O | Copper(I) oxide |
| CuO | Copper(II) oxide |
Molecular (Covalent) Compounds
| Chemical formula | Compound name |
|---|---|
| CO | Carbon monoxide |
| CO₂ | Carbon dioxide |
| SO₂ | Sulfur dioxide |
| SO₃ | Sulfur trioxide |
| NO | Nitrogen monoxide |
| NO₂ | Nitrogen dioxide |
| N₂O | Dinitrogen monoxide |
| N₂O₄ | Dinitrogen tetroxide |
| N₂O₅ | Dinitrogen pentoxide |
| PCl₃ | Phosphorus trichloride |
| PCl₅ | Phosphorus pentachloride |
| SF₆ | Sulfur hexafluoride |
| CCl₄ | Carbon tetrachloride |
| H₂O | Water |
| H₂O₂ | Hydrogen peroxide |
| NH₃ | Ammonia |
| CH₄ | Methane |
Common Acids
| Chemical formula | Compound name |
|---|---|
| HCl(aq) | Hydrochloric acid |
| HBr(aq) | Hydrobromic acid |
| HNO₃(aq) | Nitric acid |
| HNO₂(aq) | Nitrous acid |
| H₂SO₄(aq) | Sulfuric acid |
| H₂SO₃(aq) | Sulfurous acid |
| H₃PO₄(aq) | Phosphoric acid |
| H₂CO₃(aq) | Carbonic acid |
| HC₂H₃O₂(aq) | Acetic acid |
Final reminder: A chemical formula is packed with information, but some molecular formulas can represent more than one substance. For example, C₆H₁₂O₆ can represent glucose or fructose. For the common ionic compounds and simple molecular compounds in this guide, however, these naming rules will take you a long way.
