Difficulty: Introductory | Prerequisites: Know the common charges of main-group ions (covered in the Atomic Structure notes) and the basic layout of the periodic table.
Tags: chemical nomenclature, naming compounds, ionic compounds, covalent compounds, molecular compounds, polyatomic ions, ionic formulas, binary compounds, prefixes, CHM 11100, Purdue general chemistry
Naming and writing formulas for chemical compounds is the "vocabulary" of chemistry. Without it, you cannot read a chemical equation, follow a lab procedure or communicate results. This material splits into two main tracks: ionic compounds (metal + non-metal, or compounds involving polyatomic ions) and covalent/molecular compounds (non-metal + non-metal). Each track has its own naming rules, and mixing them up is one of the most common early mistakes in general chemistry.
Ionic compounds are named by writing the cation name first, then the anion name (with the ending changed to "-ide" for monatomic anions). Covalent compounds between two non-metals use Greek prefixes (di-, tri-, tetra-, penta-) to indicate atom counts. Balancing charges is how you write correct ionic formulas. Polyatomic ions like OH⁻, NO₃⁻, SO₄²⁻ and CO₃²⁻ must be memorised.
Ionic compound
A compound formed by the electrostatic attraction between cations (positive ions) and anions (negative ions), typically a metal combined with a non-metal. The formula must be electrically neutral: total positive charge equals total negative charge.
Covalent (molecular) compound
A compound formed when two or more non-metals share electrons. Named using Greek prefixes to indicate the number of each type of atom.
Polyatomic ion
A charged group of two or more atoms bonded together that acts as a single unit in ionic compounds. Examples: hydroxide (OH⁻), nitrate (NO₃⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), ammonium (NH₄⁺), phosphate (PO₄³⁻).
Binary compound
A compound made of exactly two different elements. Can be ionic (NaCl) or covalent (N₂O₅).
Greek prefixes for covalent naming
Mono- (1, usually omitted for the first element), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6). These are applied to both elements in the name, and the second element always ends in "-ide."
The key rule: the total positive charge must equal the total negative charge so the compound is electrically neutral.
Write the cation first, then the anion. Use subscripts to balance.
Magnesium chloride: Mg²⁺ and Cl⁻. You need two Cl⁻ to balance one Mg²⁺. Formula: MgCl₂.
Cesium sulfide: Cs⁺ and S²⁻. You need two Cs⁺ to balance one S²⁻. Formula: Cs₂S.
Be careful to distinguish sulfide (S²⁻) from sulfate (SO₄²⁻). "Cesium sulfide" is Cs₂S, not Cs₂SO₄ or CsSO₄.
Common mistakes when writing formulas:
MgNO₃ is incorrect for magnesium nitrate. Mg²⁺ needs two NO₃⁻ ions to balance, so the correct formula is Mg(NO₃)₂. Parentheses are required around the polyatomic ion when the subscript is greater than 1.
Ba(OH)₂ is correct: Ba²⁺ balanced by two OH⁻ ions.
AlPO₄ is correct: Al³⁺ balanced by PO₄³⁻ (charges cancel one-to-one).
(NH₄)₂CO₃ is correct: two NH₄⁺ ions balanced by one CO₃²⁻ ion.
Name the cation (metal) first, using the element's name unchanged.
Name the anion second:
For monatomic anions, change the ending to "-ide": chlorine becomes chloride, sulfur becomes sulfide, oxygen becomes oxide.
For polyatomic anions, use the ion's name as-is: nitrate, sulfate, hydroxide, carbonate, phosphate.
Transition metals can form more than one type of cation. In those cases, a Roman numeral in parentheses indicates the charge: iron(II) chloride = FeCl₂, iron(III) chloride = FeCl₃. This does not apply to main-group metals like Na, Mg, Al, which only form one common charge.
Mg(OH)₂ contains Mg²⁺ ions and OH⁻ ions. The hydroxide ion stays intact as a unit.
It does not break apart into O²⁻ and H⁺ or H₂. The OH⁻ is a polyatomic ion that remains together.
This is a common exam trap: when asked "what ions are present," look for the intact polyatomic ion, not its individual atoms.
Used when both elements are non-metals.
Apply Greek prefixes to indicate the number of each atom:
N₂O₅ = dinitrogen pentoxide (di- = 2 nitrogen, penta- = 5 oxygen)
NCl₃ = nitrogen trichloride (mono- is omitted on the first element, tri- = 3 chlorine)
The second element always ends in "-ide."
"Mono-" is typically dropped from the first element but kept on the second if applicable (e.g. carbon monoxide, CO).
Common errors:
Calling N₂O₅ "nitrogen oxide" or "nitrogen pentoxide" is incomplete. You must specify "dinitrogen" because there are two nitrogen atoms.
Calling N₂O₅ "dinitrogen monoxide" is wrong because there are five oxygens, not one. (Dinitrogen monoxide is N₂O, a completely different compound.)
1 = mono
2 = di
3 = tri
4 = tetra
5 = penta
6 = hexa
7 = hepta
8 = octa
9 = nona
10 = deca
Metal + non-metal (or metal + polyatomic ion) = ionic. Use ionic naming rules. No prefixes.
Non-metal + non-metal = covalent. Use Greek prefix naming rules.
Quick check: if the first element is to the left of the metalloid staircase (and is not hydrogen), the compound is ionic.
A formula is correct only if charges balance to zero.
MgNO₃ is not correct: Mg is 2+, NO₃ is 1−, total charge = +1. Correct formula is Mg(NO₃)₂.
Ba(OH)₂ is correct: Ba is 2+, two OH⁻ give 2−.
AlPO₄ is correct: Al is 3+, PO₄ is 3−.
(NH₄)₂CO₃ is correct: two NH₄⁺ give 2+, CO₃²⁻ gives 2−.
No mathematical formulas in this topic, but the charge-balance principle is the core rule:
Total positive charge = total negative charge (ionic compound must be neutral)
Subscripts indicate the number of each ion needed to achieve neutrality.
Nomenclature is the language of chemistry. Pharmacists, environmental chemists and materials scientists all need to read and write chemical names accurately. Calcium carbonate (CaCO₃) is what makes up limestone and chalk. Sodium chloride (NaCl) is table salt. Dinitrogen monoxide (N₂O) is the laughing gas used by dentists. Getting the name wrong could mean confusing a harmless compound with a dangerous one.
Students often forget to use parentheses around polyatomic ions when a subscript is needed. Mg(NO₃)₂ is correct; MgNO₃₂ is meaningless.
Students sometimes apply Greek prefixes to ionic compounds. Ionic compounds never use prefixes. You would never say "dimagnesium monoxide" for Mg₂O; the correct name is simply "magnesium oxide."
Students confuse sulfide (S²⁻) with sulfate (SO₄²⁻). "Cesium sulfide" is Cs₂S. "Cesium sulfate" would be Cs₂SO₄. The "-ate" suffix always signals a polyatomic ion containing oxygen.
When asked for ions present in a compound like Mg(OH)₂, students sometimes break OH⁻ into separate O²⁻ and H⁺. The hydroxide ion is a single unit.
⚠️ Expect at least one question asking you to write the formula from a compound name, and at least one going the other direction (name from formula).
⚠️ Questions identifying incorrect ionic formulas (like MgNO₃) are common. Check charges.
⚠️ The distinction between ionic and covalent naming is tested directly. If you see two non-metals, use prefixes. If you see a metal and a non-metal, do not.
⚠️ Know your polyatomic ions cold: hydroxide, nitrate, sulfate, carbonate, phosphate, ammonium at minimum.
⚠️ "What ions are present in this compound?" questions test whether you recognise intact polyatomic ions.
True or false: N₂O₅ is called "nitrogen pentoxide."
Fill in the blank: the formula for magnesium chloride is ______.
True or false: NCl₃ is an ionic compound.
Fill in the blank: the ions present in Ca(OH)₂ are ______ and ______.
True or false: (NH₄)₂CO₃ is a correctly balanced ionic formula.
Answers: 1. False (it is "dinitrogen pentoxide," because there are two nitrogen atoms). 2. MgCl₂. 3. False (both nitrogen and chlorine are non-metals, so it is covalent). 4. Ca²⁺ and OH⁻. 5. True.
Q: What is the formula for magnesium chloride?
A: MgCl₂. Mg²⁺ needs two Cl⁻ ions to balance.
Q: What is the name for N₂O₅?
A: Dinitrogen pentoxide. Two non-metals, so use Greek prefixes: di- (2 nitrogen) + penta- (5 oxygen) + -oxide.
Q: What is the formula for cesium sulfide?
A: Cs₂S. Cesium forms Cs⁺ ions; sulfide is S²⁻. Two Cs⁺ balance one S²⁻.
Q: What are the ions present in Mg(OH)₂?
A: Mg²⁺ and OH⁻. The hydroxide ion stays together as a single polyatomic unit.
Q: Which of these ionic compound formulas is incorrect: MgNO₃, Ba(OH)₂, AlPO₄ or (NH₄)₂CO₃?
A: MgNO₃. Magnesium is 2+ and nitrate is 1−, so the formula needs to be Mg(NO₃)₂ to balance charges.
Q: What is the name for NCl₃?
A: Nitrogen trichloride. Mono- is dropped from the first element; tri- indicates three chlorine atoms.
Q: How can you tell whether to use Greek prefixes or not when naming a compound?
A: If both elements are non-metals, use Greek prefixes (covalent naming). If the compound contains a metal and a non-metal (or a polyatomic ion), use ionic naming with no prefixes.
Nomenclature feeds directly into writing and balancing chemical equations (you need correct formulas). Understanding ionic vs. covalent also sets the stage for the bonding chapter, where you learn why some atoms share electrons and others transfer them entirely. Polyatomic ions reappear in acid-base chemistry (nitric acid = HNO₃, sulfuric acid = H₂SO₄) and solubility rules.
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