Difficulty: Introductory to Intermediate | Prerequisites: Binary ionic naming (Study Notes Part 1), periodic table familiarity.
Once you move past simple binary ionic compounds, two complications arise: metals that can form more than one ion (transition metals such as iron, copper, cobalt, chromium, and lead), and anions made of multiple atoms bonded together (polyatomic ions such as sulfate, chromate, acetate, and hypochlorite). This is the layer of nomenclature that trips students up most often, because you now need to memorise a set of polyatomic ions and learn when Roman numerals are required. If you have binary ionic naming down, the underlying logic is the same. The new pieces are just a lookup table of polyatomic ions and the Roman-numeral convention.
Transition metals need Roman numerals in parentheses to show which charge the metal is using (e.g. Copper (II) Bromide). Polyatomic ions are named as a unit, not element by element. Charge balancing works the same way as with binary ionic compounds, but if you need more than one polyatomic ion, wrap it in parentheses before adding the subscript.
Transition metal
A metal from the d-block of the periodic table (groups 3 to 12) that can form ions with more than one charge. Think of it as a metal that can't make up its mind about how many electrons to lose. Common examples: iron (Fe²⁺ or Fe³⁺), copper (Cu⁺ or Cu²⁺), cobalt (Co²⁺ or Co³⁺), chromium (Cr²⁺ or Cr³⁺), lead (Pb²⁺ or Pb⁴⁺).
Stock notation (Roman-numeral system)
The convention of writing the metal's charge as a Roman numeral in parentheses immediately after the metal name. In simple terms, it tells you which version of the metal you are dealing with. Example: Iron (III) means Fe³⁺.
Polyatomic ion
An ion made of two or more atoms covalently bonded together that carries an overall charge. In simple terms, it is a group of atoms that acts as a single ion. You need to memorise the common ones.
Sulfate (SO₄²⁻)
A polyatomic anion. Not to be confused with sulfite (SO₃²⁻), which has one fewer oxygen.
Chromate (CrO₄²⁻)
A polyatomic anion containing chromium and oxygen.
Acetate (C₂H₃O₂⁻)
A polyatomic anion, sometimes written CH₃COO⁻. The organic-looking formula is the same ion.
Hydrogen sulfite / bisulfite (HSO₃⁻)
A polyatomic anion. The "hydrogen" prefix means one H⁺ is attached to the sulfite ion.
Hypochlorite (ClO⁻)
A polyatomic anion with one chlorine and one oxygen. The "hypo-" prefix means one fewer oxygen than chlorite.
Use Roman numerals for metals that can have more than one charge. These are mostly transition metals (Fe, Cu, Co, Cr, Mn, etc.) plus a few main-group exceptions like lead (Pb) and tin (Sn).
Do not use Roman numerals for metals with only one common charge: all Group 1 metals (+1), all Group 2 metals (+2), aluminium (+3), zinc (+2), silver (+1).
The Roman numeral tells the reader the charge on the metal cation, not the number of atoms.
Write the metal name, then the Roman numeral in parentheses, then the anion name.
To figure out the Roman numeral from a formula, work backwards from the anion's known charge.
Fe₂O₃: oxygen is −2, three oxygens give −6 total. Two iron atoms share +6, so each iron is +3. Name: Iron (III) Oxide.
Cr₂O₃: same logic. Chromium (III) Oxide.
CoSO₄: sulfate is SO₄²⁻ (charge −2). One cobalt balances to +2. Name: Cobalt (II) Sulfate.
Recognise the polyatomic ion as a unit. The compound Na₂SO₄ contains Na⁺ and SO₄²⁻, not separate S and O atoms.
Name the cation first, then the polyatomic ion by its memorised name. No "-ide" ending unless the polyatomic ion's own name already ends that way (e.g. hydroxide, cyanide).
If the compound contains the ammonium ion (NH₄⁺), that is the cation. NH₄I is ammonium iodide.
Balance charges as usual with the criss-cross method.
If you need more than one polyatomic ion, place it in parentheses before adding the subscript.
Magnesium acetate: Mg²⁺ and C₂H₃O₂⁻. Criss-cross gives Mg(C₂H₃O₂)₂.
Lithium hydrogen sulfite: Li⁺ and HSO₃⁻. Charges already balance 1:1. Formula: LiHSO₃.
Copper (II) Bromide: Cu²⁺ and Br⁻. Criss-cross: CuBr₂.
Sodium Sulfate: Na⁺ and SO₄²⁻. Criss-cross: Na₂SO₄.
Lead (II) Chromate: Pb²⁺ and CrO₄²⁻. Charges balance 1:1. Formula: PbCrO₄.
Calcium Fluoride: Ca²⁺ and F⁻. Criss-cross: CaF₂.
Lithium Hydrogen Sulfite: Li⁺ and HSO₃⁻. Formula: LiHSO₃.
NH₄I: Ammonium (NH₄⁺) and iodide (I⁻). Name: Ammonium Iodide.
KClO: Potassium (K⁺) and hypochlorite (ClO⁻). Name: Potassium Hypochlorite.
Mg(C₂H₃O₂)₂: Magnesium and acetate. Name: Magnesium Acetate.
Fe₂O₃: Iron (III) Oxide (see reasoning above).
Students often put Roman numerals on metals that do not need them. Sodium sulfate is just "sodium sulfate," not "sodium (I) sulfate." Sodium only forms +1 ions.
Confusing sulfate (SO₄²⁻) with sulfite (SO₃²⁻), and sulfide (S²⁻). These are three different anions. Sulfide is a single atom; sulfate and sulfite are polyatomic.
Forgetting parentheses around polyatomic ions when the subscript is greater than 1. Mg(C₂H₃O₂)₂ is correct; MgC₂H₃O₂₂ is nonsensical.
Reading "Iron (III) Oxide" as "three atoms of iron." The Roman numeral is the charge per iron atom, not the count of atoms.
⚠️ Expect questions that give you a formula like CoSO₄ and ask you to name it, including the correct Roman numeral. You must work backwards from the anion charge to find the metal's charge.
⚠️ A common exam format: a mixed list of compounds where you must decide whether each needs Roman numerals, Greek prefixes, or neither. Classification is the first step before naming.
⚠️ Memorise the common polyatomic ions (sulfate, nitrate, carbonate, phosphate, hydroxide, acetate, chromate, hypochlorite, and their relatives). Many exams test these directly.
True or False: Zinc chloride requires a Roman numeral in its name. (Answer: False. Zinc always forms Zn²⁺.)
Fill in the blank: The Roman numeral in Cobalt (II) Sulfate tells you that cobalt has a charge of ____. (Answer: +2.)
True or False: In the formula Mg(C₂H₃O₂)₂, the parentheses are optional. (Answer: False. Without them the subscript 2 would apply only to the last atom, not the whole polyatomic ion.)
Fill in the blank: The polyatomic ion ClO⁻ is called ____. (Answer: Hypochlorite.)
True or False: NH₄I is named "nitrogen hydrogen iodide." (Answer: False. NH₄⁺ is the ammonium ion, so the name is ammonium iodide.)
Q: Name the compound Fe₂O₃.
A: Oxygen is −2, three oxygens total −6. Two irons share +6, so each is +3. Name: Iron (III) Oxide.
Q: Write the formula for Copper (II) Bromide.
A: Cu²⁺ and Br⁻. Criss-cross: CuBr₂.
Q: Name the compound CoSO₄.
A: Sulfate is SO₄²⁻ with charge −2. One cobalt ion balances at +2. Name: Cobalt (II) Sulfate.
Q: Write the formula for magnesium acetate.
A: Mg²⁺ and C₂H₃O₂⁻. Two acetate ions needed. Formula: Mg(C₂H₃O₂)₂.
Q: A student names KClO as "potassium chlorine oxide." What went wrong?
A: ClO⁻ is the polyatomic ion hypochlorite. It should be named as a unit: potassium hypochlorite. The student broke it into separate elements, which only applies to binary compounds.
Q: Name the compound Cr₂O₃.
A: Oxygen is −2, three oxygens total −6. Two chromiums share +6, so each is +3. Name: Chromium (III) Oxide.
This builds directly on binary ionic naming. If you are comfortable with charge balancing from Part 1, the only new material here is the polyatomic-ion table and the Roman-numeral rule.
Polyatomic ions reappear in acid naming (H₂SO₄ is sulfuric acid because it contains sulfate), in net ionic equations, and in solubility rules. The better you know these ions now, the less memorisation you face later.
Transition-metal charges also come up in redox reactions and electrochemistry, where tracking oxidation states is the central skill.
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