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B. Matter, Chemical Trends, and Chemical Bonding · Matter, Chemical Trends, and Chemical Bonding

IUPAC Names and Formulas: Molecular Compounds and Acids

B2.7 A1.12About 65 minutes
Learning goals
  • I can name and write formulas for binary molecular compounds using prefixes.
  • I can name and write formulas for binary acids and oxyacids, including per- and hypo- acids.
  • I can classify a compound as ionic, molecular or an acid before naming it.
  • I can correct names that use the wrong set of rules.

Words to know

binary molecular compound
A compound of two non-metal elements held together by covalent bonds.
prefix
A part added to the start of a name; in molecular names it gives the number of atoms (mono-, di-, tri-...).
acid
For naming purposes, a compound that releases H⁺ ions in water; its formula starts with H and is written with (aq).
binary acid
An acid made of hydrogen and one other element, named hydro-...-ic acid.
oxyacid
An acid containing hydrogen, oxygen and another element, named from its oxyanion (-ic or -ous).
diatomic element
An element whose molecules have two atoms: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Dihydrogen monoxide alert!

Every few years a prank warning goes around: "Dihydrogen monoxide is found in acid rain, causes severe burns as a gas, and is in nearly every Canadian lake!" All true, and all about water, H₂O. The joke works because a systematic name sounds scary to people who do not know the rules.

This lesson finishes the naming rules from expectation B2.7 with molecular compounds and acids, and shows you how to look at any formula and decide which set of rules to use.

Remember from Grade 10

Molecular compounds (two non-metals) use prefixes to count atoms, and are never reduced: N₂O₄ is dinitrogen tetroxide. Acids are named from their negative ion: chloride → hydrochloric acid; nitrate → nitric acid; nitrite → nitrous acid.

Naming binary molecular compounds

  1. Name the first element in the formula (usually the one farther left or lower in the periodic table) in full.
  2. Name the second element with its stem + -ide.
  3. Add a prefix to each name for the number of atoms: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, nona 9, deca 10.
  4. Never use mono- on the first element (CO is carbon monoxide, not monocarbon monoxide).
  5. When a prefix ending in a or o comes before oxide, drop that vowel: monoxide, tetroxide, pentoxide (but dioxide and trioxide keep theirs).

Some molecular compounds keep common names that you must know: H₂O water, NH₃ ammonia, CH₄ methane, H₂O₂ hydrogen peroxide, O₃ ozone.

Also remember the elements that exist as molecules: the diatomic elements H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂, plus P₄ and S₈.

COCOcarbon monoxideOCOCO₂carbon dioxideNNON₂Odinitrogen monoxideClPClClPCl₃phosphorus trichlorideThe prefixes count the atoms in one molecule (shapes not shown)
In molecular compounds the prefixes tell you exactly how many atoms of each element are in one molecule.

Naming acids

For naming, an acid is a compound that releases hydrogen ions (H⁺) when dissolved in water. Its formula usually starts with H (acetic acid, CH₃COOH, is an exception), and the state symbol (aq) shows it is in solution. (Pure HCl(g) is called hydrogen chloride; only HCl(aq) is hydrochloric acid.) Name the acid from its negative ion:

  • -ide ion → hydro- + stem + -ic acid: HBr(aq), hydrobromic acid; H₂S(aq), hydrosulfuric acid.
  • -ate ion → stem + -ic acid: H₂SO₄(aq), sulfuric acid; H₃PO₄(aq), phosphoric acid; CH₃COOH(aq), acetic acid.
  • -ite ion → stem + -ous acid: H₂SO₃(aq), sulfurous acid; HNO₂(aq), nitrous acid.
  • per- and hypo- prefixes stay: HClO₄(aq) perchloric acid, HClO(aq) hypochlorous acid.

Some textbooks also use names such as aqueous hydrogen chloride for HCl(aq) and aqueous hydrogen sulfate for H₂SO₄(aq). The traditional acid names are still the most common.

Negative ionAcid name-ide ion (no O)Cl⁻ chloridehydro- ... -ic acidHCl(aq) hydrochloric acidper- ... -ate ionClO₄⁻ perchlorateper- ... -ic acidHClO₄(aq) perchloric acid-ate ionClO₃⁻ chlorate-ic acidHClO₃(aq) chloric acid-ite ionClO₂⁻ chlorite-ous acidHClO₂(aq) chlorous acidhypo- ... -ite ionClO⁻ hypochloritehypo- ... -ous acidHClO(aq) hypochlorous acidMemory aid:I ate somethingic-ky, but the bitewas delicious.
Acid names come from the negative ion. The chlorine family shows every pattern, including per- and hypo-.

Which rules do I use? Classify first

Before you name anything, decide what kind of compound it is:

  1. Does the formula start with H and have (aq) (or does the question say it is dissolved in water)? → acid rules.
  2. Does it contain a metal or the ammonium ion (NH₄⁺)? → ionic rules (no prefixes; Roman numerals for multivalent metals; hydrate prefixes only for water).
  3. Is it made of non-metals only? → molecular rules (prefixes).

The most common mistake in this whole unit is using the wrong set of rules, such as "magnesium dichloride" (prefixes on an ionic compound) or "nitrogen(IV) oxide" (Roman numerals on a molecular compound).

Worked example

Example 1: Molecular names and formulas

(a) Name P₂O₅, SF₆ and N₂O₃. (b) Write formulas for carbon tetrachloride and diphosphorus trisulfide.

  1. (a) P₂O₅: di + phosphorus, pent(a) + oxide → diphosphorus pentoxide.
  2. SF₆: no mono on the first element → sulfur hexafluoride.
  3. N₂O₃: dinitrogen trioxide (tri keeps its i).
  4. (b) carbon (1) tetra (4) chloride → CCl₄. di (2) phosphorus, tri (3) sulfide → P₂S₃.

Diphosphorus pentoxide, sulfur hexafluoride, dinitrogen trioxide; CCl₄ and P₂S₃.

Worked example

Example 2: Naming acids

Name (a) HI(aq), (b) HNO₃(aq), (c) H₃PO₃(aq) (from the phosphite ion, PO₃³⁻), (d) HBrO₄(aq).

  1. (a) Iodide (-ide) → hydroiodic acid.
  2. (b) Nitrate (-ate) → nitric acid.
  3. (c) Phosphite (-ite) → phosphorous acid (careful: phosphorous, with the -ous ending).
  4. (d) BrO₄⁻ is perbromate (one more O than bromate, BrO₃⁻) → perbromic acid.

Hydroiodic, nitric, phosphorous and perbromic acid.

Worked example

Example 3: Classify, then name

Name each compound: (a) NO₂, (b) Ni(NO₃)₂, (c) H₂CO₃(aq), (d) NH₄Cl, (e) Cl₂O₇.

  1. (a) Non-metals only → molecular: nitrogen dioxide.
  2. (b) Contains a metal → ionic. Two NO₃⁻ give 2−, so Ni²⁺: nickel(II) nitrate.
  3. (c) H + (aq) → acid. Carbonate (-ate) → carbonic acid.
  4. (d) Contains NH₄⁺ → ionic: ammonium chloride (no prefixes even though it has only non-metals).
  5. (e) Non-metals only → molecular: dichlorine heptoxide.

Classify first, then use the matching rules.

Just for fun

Dinitrogen monoxide, N₂O, has a famous nickname: laughing gas. Dentists use it to help patients relax. It may be the only chemical name with a joke already built in.

Hands-on activity

Naming speed-sort game

You need: 24 small pieces of paper or index cards, a pencil, a timer (a phone works), and a family member to play against or to time you.

  1. Write one formula on each card. Use a mix of 8 molecular compounds (e.g. CO, SO₃, N₂O₅, PCl₅, SiF₄, OF₂, CS₂, H₂O₂), 8 ionic compounds (e.g. KMnO₄, CuO, (NH₄)₂SO₄, Fe(OH)₃, NaClO, SnCl₄, CaCO₃, MgSO₄·7H₂O) and 8 acids (e.g. HF(aq), H₂SO₄(aq), H₂SO₃(aq), HNO₂(aq), HClO₄(aq), HClO(aq), CH₃COOH(aq), H₃PO₄(aq)).
  2. Shuffle. Start the timer. Sort the cards into three piles: molecular, ionic, acid.
  3. Then go through each pile and say the name aloud. Your partner checks with the answers below. Add 5 seconds for each mistake.
  4. Play again tomorrow and try to beat your time.

Answers: carbon monoxide, sulfur trioxide, dinitrogen pentoxide, phosphorus pentachloride, silicon tetrafluoride, oxygen difluoride, carbon disulfide, hydrogen peroxide; potassium permanganate, copper(II) oxide, ammonium sulfate, iron(III) hydroxide, sodium hypochlorite, tin(IV) chloride, calcium carbonate, magnesium sulfate heptahydrate; hydrofluoric, sulfuric, sulfurous, nitrous, perchloric, hypochlorous, acetic and phosphoric acid.

Your turn

1

Name: (a) CS₂, (b) N₂O, (c) BF₃, (d) S₂Cl₂.

2

Write formulas: (a) dinitrogen pentoxide, (b) iodine heptafluoride, (c) silicon dioxide.

3

Name: (a) HF(aq), (b) H₂SO₄(aq), (c) HClO₂(aq).

4

Write formulas: (a) hypochlorous acid, (b) nitrous acid, (c) hydrosulfuric acid.

5

Classify and name: (a) SO₂, (b) Na₂SO₃, (c) H₂SO₃(aq).

6

Why is CO named carbon monoxide and not monocarbon monoxide?

More practice: warm-up

1

What prefix means 5?

2

What is the common name of NH₃?

3

Name HCl(aq).

4

List the seven diatomic elements.

5

Which kind of compound uses prefixes: ionic or molecular?

More practice: on your own

1

Correct each wrong name: (a) MgCl₂ "magnesium dichloride", (b) NO₂ "nitrogen(IV) oxide", (c) H₂SO₄(aq) "hydrosulfuric acid".

2

Bromate is BrO₃⁻. Name HBrO₂(aq) and HBrO(aq).

3

Write the formula of phosphoric acid and explain how many H atoms it has.

4

Name P₄O₁₀ and explain why it is not simplified to P₂O₅.

5

Explain why HCl(g) and HCl(aq) have different names.

Challenge

1

Iodic acid is HIO₃(aq). Write the formulas of periodic acid, iodous acid and hypoiodous acid, and the names of their negative ions.

2

A student writes the formula of "chromic acid" as HCrO₄(aq). Using the chromate ion (CrO₄²⁻), find the error and give the correct formula.

3

Name these and state which is the odd one out and why: N₂O₄, (NH₄)₂S, H₂S(aq), CCl₄.

Check your understanding

Quiz

1.

What is the name of N₂O₅?

2.

What is the formula of sulfur trioxide?

3.

HNO₂(aq) is named

4.

HClO₄(aq) is named

5.

Which name uses the correct rules?

6.

Which formula should be named with prefixes?

For parents and teachers

Big idea: This lesson completes expectation B2.7 (IUPAC names and formulas) with molecular compounds and acids, and practises the key decision: classify the compound first, then use the matching naming rules.

How to help: Play the speed-sort game together. Being fast at classifying (molecular, ionic or acid) prevents most naming errors.

Common difficulties:

  • Using prefixes on ionic compounds or Roman numerals on molecular ones.
  • Adding "hydro-" to oxyacids (H₂SO₄ is sulfuric acid, not hydrosulfuric acid).
  • Forgetting that per- and hypo- carry through to acid names.
  • Forgetting that NH₄⁺ compounds are ionic even though they contain no metal.