B. Matter, Chemical Trends, and Chemical Bonding · Matter, Chemical Trends, and Chemical BondingIUPAC Names and Formulas: Ionic Compounds
B2.7 A1.12About 70 minutes
Learning goals- I can name and write formulas for ionic compounds with multivalent metals and polyatomic ions.
- I can recognize classical (-ous/-ic) names and convert them to Stock names.
- I can use the per-, -ate, -ite, hypo- pattern to name oxyanions.
- I can name and write formulas for hydrates.
Words to know
IUPAC
The International Union of Pure and Applied Chemistry, which sets the rules for chemical names.
Stock system
Naming multivalent metal ions with a Roman numeral for the charge, e.g. iron(III).
classical name
An older name using -ous (lower charge) and -ic (higher charge), e.g. ferrous, ferric.
oxyanion
A polyatomic negative ion that contains oxygen, e.g. sulfate or chlorite.
hydrate
An ionic compound with a definite number of water molecules in its crystal, e.g. CuSO₄·5H₂O.
anhydrous
Without water; the compound left when a hydrate's water is removed.
Names that tell you the formula
Read the label on a tube of toothpaste and you might see stannous fluoride. A bag of bath salts says magnesium sulfate heptahydrate. A bottle of bleach lists sodium hypochlorite. Each name follows rules set by IUPAC, the International Union of Pure and Applied Chemistry, so that a chemist in Sudbury, Shanghai or São Paulo writes exactly the same formula from it.
You learned the basic rules in Grade 10. This lesson reviews them quickly and adds the remaining pieces for ionic compounds: classical names, families of oxyanions with per- and hypo-, and hydrates.
Remember from Grade 10
Binary ionic compounds: metal name + non-metal stem + -ide (CaF₂, calcium fluoride). Multivalent metals use a Roman numeral for the ion's charge, the Stock system (FeCl₃, iron(III) chloride). Polyatomic ions keep their names and go in brackets when there is more than one (Ca(OH)₂, calcium hydroxide). The total positive charge must equal the total negative charge.
Common multivalent metals and their classical names
The Stock system (Roman numerals) is the IUPAC method. Older classical names are still seen on labels and in older books. They use the Latin root of the metal with -ous for the lower charge and -ic for the higher charge. You should be able to recognize them.
| Metal | Ions | Stock names | Classical names |
|---|
| iron | Fe²⁺, Fe³⁺ | iron(II), iron(III) | ferrous, ferric |
| copper | Cu⁺, Cu²⁺ | copper(I), copper(II) | cuprous, cupric |
| tin | Sn²⁺, Sn⁴⁺ | tin(II), tin(IV) | stannous, stannic |
| lead | Pb²⁺, Pb⁴⁺ | lead(II), lead(IV) | plumbous, plumbic |
| cobalt | Co²⁺, Co³⁺ | cobalt(II), cobalt(III) | cobaltous, cobaltic |
| gold | Au⁺, Au³⁺ | gold(I), gold(III) | aurous, auric |
Polyatomic ions you need
Know the names, formulas and charges in this table. Many come in families that differ only in the number of oxygen atoms.
| Charge 1+ | Charge 1− | Charge 2− | Charge 3− |
|---|
| ammonium NH₄⁺ | hydroxide OH⁻ | carbonate CO₃²⁻ | phosphate PO₄³⁻ |
| nitrate NO₃⁻ / nitrite NO₂⁻ | sulfate SO₄²⁻ / sulfite SO₃²⁻ | |
| hydrogen carbonate HCO₃⁻ | hydrogen phosphate HPO₄²⁻ | |
| hydrogen sulfate HSO₄⁻ | chromate CrO₄²⁻ | |
| dihydrogen phosphate H₂PO₄⁻ | dichromate Cr₂O₇²⁻ | |
| acetate CH₃COO⁻ | peroxide O₂²⁻ | |
| permanganate MnO₄⁻ | | |
| cyanide CN⁻ | | |
| perchlorate ClO₄⁻, chlorate ClO₃⁻, chlorite ClO₂⁻, hypochlorite ClO⁻ | | |
Oxyanion families: per-, -ate, -ite, hypo-
An oxyanion is a polyatomic ion containing oxygen. When one element forms several oxyanions with the same charge, the names follow a pattern based on the -ate ion:
- per-...-ate: one more oxygen than -ate (perchlorate, ClO₄⁻)
- -ate: the reference ion (chlorate, ClO₃⁻)
- -ite: one fewer oxygen than -ate (chlorite, ClO₂⁻)
- hypo-...-ite: two fewer oxygens than -ate (hypochlorite, ClO⁻)
The charge stays the same throughout the family. So once you know chlorate is ClO₃⁻, you can work out all the others. The same pattern works for bromine and iodine (bromate BrO₃⁻, periodate IO₄⁻, hypobromite BrO⁻).
Hydrates
Some ionic crystals trap a definite number of water molecules in their lattice. These compounds are called hydrates. In the formula, the water is written after a raised dot, and the name uses a Greek prefix + hydrate:
- CuSO₄·5H₂O: copper(II) sulfate pentahydrate (bright blue crystals)
- MgSO₄·7H₂O: magnesium sulfate heptahydrate (Epsom salt)
- CaSO₄·2H₂O: calcium sulfate dihydrate (gypsum, used to make drywall)
- Na₂CO₃·10H₂O: sodium carbonate decahydrate (washing soda)
The dot means "combined with"; it is not multiplication. Heating a hydrate drives off the water and leaves the anhydrous compound ("without water"). Blue copper(II) sulfate pentahydrate turns into white anhydrous copper(II) sulfate, CuSO₄. Prefixes: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, nona 9, deca 10.
Worked exampleExample 1: Naming with a multivalent metal and a polyatomic ion
Name (a) Fe₂(SO₄)₃, (b) Cu(NO₂)₂, (c) SnO₂.
- (a) Sulfate is SO₄²⁻; three of them give 6−. Two Fe must give 6+, so each is Fe³⁺: iron(III) sulfate (classical: ferric sulfate).
- (b) Nitrite is NO₂⁻; two give 2−. One Cu must be Cu²⁺: copper(II) nitrite.
- (c) Two O²⁻ give 4−. One Sn is Sn⁴⁺: tin(IV) oxide (stannic oxide). Do not "un-crisscross" to get Sn²⁺: always check charges.
(a) iron(III) sulfate; (b) copper(II) nitrite; (c) tin(IV) oxide.
Worked exampleExample 2: Writing formulas
Write formulas for (a) ammonium phosphate, (b) lead(II) acetate, (c) sodium hypochlorite, (d) potassium permanganate.
- (a) NH₄⁺ and PO₄³⁻: need 3 NH₄⁺ for one PO₄³⁻ → (NH₄)₃PO₄.
- (b) Pb²⁺ and CH₃COO⁻: need 2 acetates → Pb(CH₃COO)₂.
- (c) Na⁺ and ClO⁻: 1 : 1 → NaClO (the active ingredient in household bleach).
- (d) K⁺ and MnO₄⁻: 1 : 1 → KMnO₄.
(NH₄)₃PO₄, Pb(CH₃COO)₂, NaClO, KMnO₄.
Worked exampleExample 3: Hydrates
(a) Name CoCl₂·6H₂O. (b) Write the formula of nickel(II) sulfate heptahydrate.
- (a) Co with two Cl⁻ is Co²⁺: cobalt(II) chloride. Six waters: hexahydrate → cobalt(II) chloride hexahydrate.
- (b) Ni²⁺ and SO₄²⁻: NiSO₄. Hepta = 7 → NiSO₄·7H₂O.
(a) cobalt(II) chloride hexahydrate; (b) NiSO₄·7H₂O.
Just for funIron's symbol, Fe, comes from its Latin name, ferrum, which is also where the classical names "ferrous" and "ferric" come from. Tin's symbol, Sn, comes from Latin stannum, so stannous fluoride in toothpaste is tin(II) fluoride. Chemistry is secretly a Latin class.
Hands-on activityLabel detective: ionic compounds at home
You need: a notebook and access to labels of products around your home (toothpaste, antacid tablets, Epsom salt, baking soda, water-softener salt, plant fertilizer, vitamins). Read labels only; do not open, mix or taste any product, and do not handle cleaning products: ask a grown-up to show you the label.
- Find at least six ionic compounds listed on labels. Common finds: sodium fluoride or stannous fluoride (toothpaste), calcium carbonate (antacids), magnesium sulfate (Epsom salt), sodium bicarbonate (baking soda), potassium chloride (salt substitute or water softener), ammonium nitrate or potassium nitrate (some fertilizers), zinc oxide (sunscreen), ferrous sulfate or ferrous fumarate (iron supplements).
- For each, write the IUPAC name (convert classical names, e.g. stannous fluoride → tin(II) fluoride; sodium bicarbonate → sodium hydrogen carbonate) and the chemical formula.
- Identify which contain polyatomic ions and which contain multivalent metals.
What to look for: SnF₂, NaF, CaCO₃, MgSO₄ (often MgSO₄·7H₂O), NaHCO₃, KCl, NH₄NO₃ or KNO₃, ZnO, FeSO₄. (Fumarate is an organic ion you have not studied; just note it.)
Your turn
1Name: (a) K₂SO₃, (b) Al(OH)₃, (c) NH₄HCO₃.
(a) potassium sulfite; (b) aluminium hydroxide; (c) ammonium hydrogen carbonate.
2Name: (a) CuCl, (b) Fe(NO₃)₂, (c) PbO₂.
(a) copper(I) chloride; (b) iron(II) nitrate; (c) lead(IV) oxide.
3Write formulas: (a) calcium phosphate, (b) iron(III) hydroxide, (c) sodium perchlorate.
(a) Ca₃(PO₄)₂; (b) Fe(OH)₃; (c) NaClO₄.
4Give the formula and charge of the bromite ion, given bromate is BrO₃⁻.
-ite has one fewer O: BrO₂⁻.
5Name MgSO₄·7H₂O and explain what the dot means.
magnesium sulfate heptahydrate; the dot means 7 water molecules are combined in the crystal for each MgSO₄ (not multiplication).
6Convert to Stock names: (a) cupric oxide, (b) ferrous chloride.
(a) copper(II) oxide; (b) iron(II) chloride.
More practice: warm-up
1What is the formula of the ammonium ion?
3What charge does iron have in iron(III) oxide?
4What does anhydrous mean?
Without water (a hydrate after its water has been removed).
5What is the formula of the hydroxide ion?
More practice: on your own
1Name: (a) Co₂(CO₃)₃, (b) Sn(SO₄)₂, (c) Ca(ClO)₂.
(a) 3 CO₃²⁻ = 6−, so each Co is 3+: cobalt(III) carbonate. (b) 2 SO₄²⁻ = 4−, Sn⁴⁺: tin(IV) sulfate. (c) calcium hypochlorite.
2Write formulas: (a) gold(III) chloride, (b) ammonium dichromate, (c) potassium hydrogen phosphate.
(a) AuCl₃; (b) (NH₄)₂Cr₂O₇; (c) K₂HPO₄.
3Write the formula of copper(II) sulfate pentahydrate and of the anhydrous compound formed when it is heated.
CuSO₄·5H₂O → CuSO₄ (+ 5H₂O driven off).
4A student names Fe₂O₃ as "iron(II) oxide" because of the subscript 2. Explain the error.
Three O²⁻ give 6−, so two Fe must give 6+: each is Fe³⁺. The correct name is iron(III) oxide. The Roman numeral is the ion's charge, not a subscript.
5Periodate is IO₄⁻. Give the formulas of iodate, iodite and hypoiodite.
Periodate has one more O than iodate: iodate IO₃⁻, iodite IO₂⁻, hypoiodite IO⁻.
Challenge
1Hydrogen peroxide contains the peroxide ion pattern O₂²⁻. Write the formulas of sodium peroxide and barium peroxide, and explain why sodium peroxide is not written NaO.
Na₂O₂ and BaO₂. Peroxide is the unit O₂²⁻ (two O atoms bonded together, charge 2−); it cannot be reduced to a single O, so 2 Na⁺ balance one O₂²⁻ and the formula is not simplified to NaO.
2A hydrate of sodium sulfate has the formula Na₂SO₄·xH₂O. Its name is sodium sulfate decahydrate. What is x, and how many atoms of each element are in one formula unit?
x = 10. Atoms: Na 2, S 1, O 4 + 10 = 14, H 20.
3Manganese forms MnO₂ and KMnO₄. Find the charge (or effective charge) of manganese in MnO₂, and explain why KMnO₄ is named potassium permanganate and not with a Roman numeral.
In MnO₂, two O²⁻ give 4−, so manganese is Mn⁴⁺: manganese(IV) oxide. In KMnO₄, manganese is part of the polyatomic ion MnO₄⁻ (permanganate), which keeps its own name; the positive ion is K⁺, which forms only one ion, so no Roman numeral is needed.
Quiz
1. What is the correct name of FeCl₂?
2. What is the formula of calcium nitrate?
4. Stannous fluoride is the classical name for
5. The formula CuSO₄·5H₂O represents
6. What is the formula of ammonium sulfate?
7. Which name is written correctly by IUPAC rules?
Big idea: Expectation B2.7 asks students to use IUPAC nomenclature to write chemical formulas and names, including for compounds with multivalent elements and polyatomic ions. Much of this was introduced in Grade 10; the new pieces are classical names, oxyanion families (per-/hypo-) and hydrates.
How to help: Make flashcards of the polyatomic ions (name on one side, formula and charge on the other) and quiz in short bursts. The label-detective activity is a good way to connect names to real products. Please handle any cleaning product labels yourself.
Common difficulties:
- Reading the Roman numeral as a subscript. It is the charge of one metal ion.
- Forgetting brackets around polyatomic ions (Ca(OH)₂, not CaOH₂).
- Using prefixes (di-, tri-) in ionic names. Prefixes are only for molecular compounds and hydrates.
- Treating the hydrate dot as multiplication.