Difficulty: Introductory to Intermediate | Prerequisites: Ions and basic ionic compound naming (Doc 1 of this set)
The previous set of notes covered metals with only one possible ionic charge (like Na¹⁺ or Ca²⁺). This section adds two layers of complexity. First, transition metals such as iron and copper can form ions with different charges, so you need Roman numerals in the name to specify which one. Second, many ions are not single atoms but groups of atoms bonded together that carry a charge as a unit (polyatomic ions, like sulfate or nitrate). If you can name and write formulas for basic ionic compounds, you are ready for this.
Transition metals can form more than one type of ion, so their compounds use Roman numerals to indicate the metal's charge (e.g. Iron(III) chloride means Fe³⁺ paired with Cl⁻). Polyatomic ions are groups of atoms that carry a collective charge (like CO₃²⁻ or NO₃¹⁻) and are treated as a single unit when writing formulas. Both follow the same charge-balancing rules as basic ionic compounds.
Transition metal
A metal in the d-block of the periodic table (groups 3 through 12) that can form cations with more than one possible charge.
In simple terms, these are the metals in the middle of the periodic table whose ionic charge you cannot predict from their group number alone.
Roman numeral (Stock) naming system
A naming convention where a Roman numeral in parentheses after the metal name indicates the charge of the metal ion. For example, Iron(II) means Fe²⁺ and Iron(III) means Fe³⁺.
Think of it as: the Roman numeral tells you which version of the metal ion you are dealing with.
Polyatomic ion
A group of two or more atoms covalently bonded together that carries a net electrical charge and acts as a single ion in ionic compounds.
In simple terms, it is a cluster of atoms that sticks together and behaves like one ion. Examples: sulfate (SO₄²⁻), nitrate (NO₃¹⁻), carbonate (CO₃²⁻), hydroxide (OH¹⁻), phosphate (PO₄³⁻).
Oxyanion
A polyatomic ion that contains oxygen bonded to another element. Many common polyatomic ions are oxyanions (sulfate, nitrate, carbonate, phosphate).
Think of it as: any polyatomic ion with oxygen in it.
Bicarbonate (hydrogen carbonate)
HCO₃¹⁻. A polyatomic ion formed when carbonate (CO₃²⁻) picks up a hydrogen. The "bi-" prefix (or "hydrogen" prefix) indicates the extra H.
In simple terms, it is carbonate with a hydrogen attached, giving it a 1− charge instead of 2−.
Transition metals have partially filled d-orbitals, which means they can lose different numbers of electrons depending on the compound. Iron, for instance, can be Fe²⁺ or Fe³⁺. You cannot predict the charge from the group number the way you can with Group 1A or 2A metals.
The Roman numeral in the name tells you the charge on the metal ion.
Iron(III) chloride: Fe³⁺ paired with Cl¹⁻, giving FeCl₃.
Iron(II) oxide: Fe²⁺ paired with O²⁻, giving FeO.
Copper(I) sulfide: Cu¹⁺ paired with S²⁻, giving Cu₂S. (Two Cu¹⁺ needed to balance one S²⁻.)
Copper(II) nitride: Cu²⁺ paired with N³⁻, giving Cu₃N₂.
Zinc oxide: Zn²⁺ paired with O²⁻, giving ZnO. (Zinc almost always forms Zn²⁺, so the Roman numeral is often omitted.)
Silver sulfide: Ag¹⁺ paired with S²⁻, giving Ag₂S. (Silver almost always forms Ag¹⁺, so the Roman numeral is often omitted.)
Cu₂S: two Cu ions balance one S²⁻, so each Cu is 1+. Name: Copper(I) sulfide.
Fe₂O₃: two Fe ions balance three O²⁻ (total charge 6−), so each Fe is 3+. Name: Iron(III) oxide.
CuCl₂: one Cu balances two Cl¹⁻ (total charge 2−), so Cu is 2+. Name: Copper(II) chloride.
FeS: one Fe balances one S²⁻, so Fe is 2+. Name: Iron(II) sulfide.
Ag₂O: two Ag balance one O²⁻, so each Ag is 1+. Name: Silver oxide.
FeBr₂: one Fe balances two Br¹⁻, so Fe is 2+. Name: Iron(II) bromide.
Iron(III) chloride (FeCl₃) as the hexahydrate: yellow/orange solid, density 2.90 g/cm³, melting point 37 °C. The low melting point reflects the hydrated form, not the anhydrous salt.
These are the polyatomic ions that appear in this lab and in most general chemistry courses:
Carbonate: CO₃²⁻
Bicarbonate (hydrogen carbonate): HCO₃¹⁻
Nitrate: NO₃¹⁻
Sulfate: SO₄²⁻
Sulfite: SO₃²⁻
Hydroxide: OH¹⁻
Phosphate: PO₄³⁻
The same charge-balancing rules apply. When you need more than one polyatomic ion, wrap it in parentheses and add the subscript outside.
Potassium carbonate: K¹⁺ and CO₃²⁻. Two K⁺ needed. Formula: K₂CO₃.
Sodium nitrate: Na¹⁺ and NO₃¹⁻. Charges balance one-to-one. Formula: NaNO₃.
Calcium bicarbonate: Ca²⁺ and HCO₃¹⁻. Two bicarbonates needed. Formula: Ca(HCO₃)₂. Note the parentheses around HCO₃.
Chromium(III) hydroxide: Cr³⁺ and OH¹⁻. Three hydroxides needed. Formula: Cr(OH)₃.
Lithium phosphate: Li¹⁺ and PO₄³⁻. Three lithiums needed. Formula: Li₃PO₄.
Potassium sulfate: K¹⁺ and SO₄²⁻. Two potassiums needed. Formula: K₂SO₄.
CaSO₄: Ca²⁺ and SO₄²⁻. Name: Calcium sulfate.
Al(NO₃)₃: Al³⁺ and NO₃¹⁻. Name: Aluminum nitrate.
Na₂CO₃: Na¹⁺ and CO₃²⁻. Name: Sodium carbonate.
MgSO₃: Mg²⁺ and SO₃²⁻. Name: Magnesium sulfite. (Not sulfate; sulfite is SO₃, sulfate is SO₄.)
Cu(OH)₂: Cu²⁺ and OH¹⁻. Name: Copper(II) hydroxide. (Roman numeral needed because copper has variable charge.)
Mg₃(PO₄)₂: Mg²⁺ and PO₄³⁻. Name: Magnesium phosphate.
Potassium carbonate (K₂CO₃): white, solid, crystalline; density 2.29 g/cm³; melting point 891 °C.
Polyatomic ions are everywhere in daily life. Baking soda is sodium bicarbonate (NaHCO₃). Fertilisers rely on nitrates and phosphates. Antacids often contain calcium carbonate (CaCO₃). Recognising these ions lets you read ingredient labels and chemical safety data sheets.
Students sometimes use Roman numerals for metals that only have one possible charge. Sodium chloride is never written as Sodium(I) chloride. Roman numerals are reserved for metals with variable charges.
Sulfate (SO₄²⁻) and sulfite (SO₃²⁻) are frequently mixed up. Sulfate has four oxygens, sulfite has three. The "-ate" suffix indicates more oxygen atoms than "-ite."
Students often forget the parentheses when a polyatomic ion needs a subscript. Ca(HCO₃)₂ is correct; CaHCO₃₂ is meaningless. The parentheses keep the polyatomic ion intact as a unit.
When naming from a formula, students sometimes guess the metal charge instead of calculating it from the anion charges. Always work it out: total negative charge divided by the number of metal ions gives the charge per metal ion.
⚠️ Be able to determine the charge of a transition metal from the compound formula (e.g. given Fe₂O₃, work out that Fe is 3+).
⚠️ Know when to use Roman numerals and when not to (variable-charge metals only).
⚠️ Memorise the common polyatomic ions, their formulas, and their charges.
⚠️ Use parentheses correctly when a formula requires more than one polyatomic ion.
⚠️ Distinguish sulfate from sulfite, and nitrate from nitrite (if covered in your course).
True or False: Zinc oxide is written as Zinc(II) oxide. (False. Zinc almost always forms Zn²⁺, so the Roman numeral is typically omitted.)
Fill in the blank: In FeCl₃, the charge on iron is ___. (3+)
True or False: The formula for calcium bicarbonate is CaHCO₃. (False. It is Ca(HCO₃)₂, with parentheses and a subscript 2.)
Fill in the blank: The polyatomic ion SO₃²⁻ is called ___. (Sulfite)
True or False: Roman numerals are used for all metal ions in ionic compound names. (False. Only for metals with more than one possible charge.)
Q: Write the formula for Copper(II) nitride.
A: Cu²⁺ and N³⁻. Criss-cross: Cu₃N₂.
Q: Name the compound Fe₂O₃ and explain how you determined the Roman numeral.
A: Iron(III) oxide. Three O²⁻ ions give a total charge of 6−. Divided by 2 Fe ions, each Fe must be 3+. Hence Iron(III).
Q: Write the formula for chromium(III) hydroxide.
A: Cr³⁺ and OH¹⁻. Three hydroxides needed. Formula: Cr(OH)₃.
Q: What is the difference between sulfate and sulfite?
A: Sulfate is SO₄²⁻ (four oxygens). Sulfite is SO₃²⁻ (three oxygens). Both carry a 2− charge.
Q: Name the compound Mg₃(PO₄)₂.
A: Magnesium phosphate. Mg²⁺ and PO₄³⁻. Three Mg²⁺ (total 6+) balance two PO₄³⁻ (total 6−).
Variable-charge metals connect to redox chemistry later in the course, where iron cycling between Fe²⁺ and Fe³⁺ is central to reactions like rusting. Polyatomic ions are the building blocks for acids (sulfuric acid = H₂SO₄, nitric acid = HNO₃), so recognising them now saves time when the acid-base unit arrives. The naming rules also carry over directly into more advanced inorganic chemistry.
Transition metals, variable charge, Roman numeral naming, Stock system, Iron(II), Iron(III), Copper(I), Copper(II), zinc oxide, silver sulfide, polyatomic ions, sulfate, sulfite, nitrate, carbonate, bicarbonate, hydroxide, phosphate, oxyanion, parentheses in formulas, charge balancing, ionic compound naming, Chemistry Lab 9