Teaching Chemical Nomenclature: A Lesson Sequence That Sticks

Ask a class to name MgCl₂ at the start of a unit and you will hear “magnesium chlorine”, “magnesium dichloride”, “magnesium chloride two” and, occasionally, the right answer. None of the wrong answers is random. Each one shows a student applying a rule that exists, just to the wrong kind of compound or at the wrong step. That makes nomenclature one of the most diagnosable topics in chemistry: listen to the mistakes and you can see exactly which idea is missing.

Naming is also a gatekeeper. A student who cannot turn “calcium hydroxide” into Ca(OH)₂ will struggle with equations, moles and titrations later, however well they understand the chemistry. This guide sets out a two-lesson sequence for students of roughly 13 to 16, with timings, activities that need nothing more than card and pens, the misconceptions to plan for, and a question bank.

Learning goals

By the end of the sequence students should be able to:

1. Decide whether a compound should be named with the ionic or the molecular system.
2. Name binary ionic compounds, including metals that need a Roman numeral.
3. Name ionic compounds that contain common polyatomic ions.
4. Name simple molecular compounds using Greek prefixes, dropping mono- on the first element.
5. Write a correct formula from a name, using ion charges for ionic compounds and prefixes for molecular ones.

Prior knowledge

• The difference between metals and non-metals, and where they sit on the periodic table.
• That atoms form ions by losing or gaining electrons, and the usual charges for groups 1, 2, 13, 15, 16 and 17.
• Ionic and covalent bonding in outline.

If ion charges are shaky, spend ten minutes at the start of lesson 1 on them. Naming ionic compounds depends entirely on knowing the charges. A printed table such as the common ions list is a fair support for the first few weeks; memorisation can come later.

Lesson 1: naming ionic compounds (60 minutes)

Starter: what’s in a name? (5 minutes)

Show three everyday names: sodium chloride (table salt), calcium carbonate (chalk and limestone), sodium hydrogencarbonate (baking soda). Ask students what the two halves of each name might refer to. Most will spot that the first word is a metal. Collect guesses about the second word, then leave them on the board.

Direct teaching: the two-part pattern (10 minutes)

Model the pattern with a small number of examples: positive ion first, named after the metal; negative ion second, with -ide for a single non-metal. Write the rule as a sentence students copy, not a list of exceptions. Show chlorine → chloride, oxygen → oxide, sulfur → sulfide, nitrogen → nitride, and point out that the ending signals “this is now an ion, not the element”.

Activity: ion-card jigsaw (20 minutes)

Give each pair a set of cards cut to show charges as shape: a +1 ion has one tab sticking out, a +2 ion has two, and a −1 ion has one notch, a −2 ion two notches. Pairs combine cards until the tabs and notches match exactly, then write the formula and the name. Na⁺ needs one Cl⁻; Mg²⁺ needs two Cl⁻; Al³⁺ and O²⁻ need two aluminium cards and three oxide cards.

The cards make the key idea physical: the ratio comes from the charges, so the name doesn’t need to state it. That single insight prevents most prefix errors later. Include some polyatomic ion cards (hydroxide, nitrate, sulfate, carbonate, ammonium) printed as one card, so students see that a polyatomic ion stays together. When two hydroxide cards are needed, show how the formula uses brackets: Ca(OH)₂.

Direct teaching: Roman numerals (10 minutes)

Introduce iron(II) and iron(III) with the cards: both exist, so the name must say which one is present. Stress that the numeral is the charge on the metal ion, not a count of anything else. Demonstrate by working backwards from FeCl₃ (three Cl⁻ means the iron is 3+) and from Fe₂O₃ (three O²⁻ is 6− in total, shared between two iron ions, so each is 3+). The Fe₂O₃ example is essential, because it breaks the false pattern “the numeral equals the number of non-metal atoms”. The oxidation number calculator is a quick way for students to check their own reasoning afterwards.

Plenary: fix the name (15 minutes)

Display ten names with errors built in (“sodium chlorine”, “calcium dihydroxide”, “iron chloride” for FeCl₃, “copper(II) oxide” for Cu₂O). Pairs correct each one and state which rule was broken. Collect the most common wrong reasons; they tell you what to reteach.

Lesson 2: molecular compounds and choosing the system (60 minutes)

Starter: two oxides of carbon (5 minutes)

Show CO and CO₂ and ask why “carbon oxide” can’t be a good enough name. Students should realise the name must now say how many oxygen atoms there are, because non-metals are not locked into one ratio by charges.

Direct teaching: prefixes (10 minutes)

Introduce mono- to deca-, the rule that mono- is dropped on the first element, and the vowel dropped before “oxide” in school spelling (monoxide, tetroxide). Work through CO₂, SO₃, PCl₅, N₂O₄ and SF₆ aloud. Mention the traditional names students must simply know: water, ammonia, methane.

Activity: sorting and naming relay (25 minutes)

Give each group an envelope of 24 formula cards mixing the two types: NaCl, CO₂, K₂O, NH₄Cl, N₂O₄, CuSO₄, SF₆, CCl₄, Al₂O₃, P₂O₅, FeO, HCl and so on. Round one: sort into “ionic naming” and “molecular naming” piles, and write the rule you used on the envelope. Round two: name every card, with one student at a time writing on the group’s sheet before passing the pen on. Award a point for each correct name and a bonus for correctly placing NH₄Cl, which has no metal but is still ionic.

A side-by-side reference such as naming ionic vs molecular compounds makes a good handout at the end of this activity, once students have tried to find the pattern themselves.

Formula from name (15 minutes)

Reverse the process. Read out names; students write formulas on mini whiteboards. For ionic names they must write the ions with charges first, then balance; for molecular names they read the numbers directly from the prefixes. Deliberately mix the two so they have to decide each time. Writing chemical formulas covers the balancing step in more depth for students who need it.

Exit ticket (5 minutes)

Three items: name K₂S, name N₂O, write the formula for copper(II) nitrate.

Misconceptions to expect

What students say or write	What’s going on	What to do
“Magnesium dichloride”	Applying molecular prefixes to an ionic compound	Return to the ion cards: the charges already fix the ratio
“Sodium chlorine”	Not changing the non-metal name to show it is an ion	Pair every element name with its ion name on a two-column list
“Iron(III) oxide has three oxygens”	Reading the Roman numeral as a count	Work through Fe₂O₃ and PbO₂ explicitly
“Sulfide, sulfite and sulfate are the same thing”	Treating the ending as decoration	Show the three formulas together: S²⁻, SO₃²⁻, SO₄²⁻
“Monocarbon monoxide”	Applying mono- everywhere	State the rule: mono- is never used on the first element
“CaOH₂” for calcium hydroxide	Not seeing a polyatomic ion as one unit	Use single cards for polyatomic ions and insist on brackets for more than one
“NH₄Cl is covalent because there’s no metal”	Using the metal test too literally	Introduce ammonium as the positive ion that acts like a metal ion
“Water is dihydrogen monoxide”	Over-applying the systematic rules	Enjoy the joke, then agree that traditional names win for water, ammonia and methane

Differentiation

Support

• Keep the ion cards and a printed charge table available for the whole unit.
• Restrict early questions to group 1 and 2 metals with halides and oxides, then add polyatomic ions, then transition metals.
• Give a four-box flowchart: metal present? → ionic → variable charge? → numeral; non-metals only → prefixes.

Stretch

• Name compounds with less familiar ions, such as dichromate, permanganate and phosphate.
• Explain why aluminium chloride and tin(IV) chloride are named the ionic way even though their bonding has strong covalent character.
• Research one compound whose traditional and systematic names both appear on labels (for example, sodium hydrogencarbonate and bicarbonate of soda) and explain the difference.
• Try naming hydrates such as CuSO₄·5H₂O.

Questions to ask during the lessons

• “How do you know the ratio in this compound without anyone telling you?”
• “Why does the name of MgCl₂ not need a number, but the name of N₂O₄ does?”
• “What does the III in iron(III) tell you? What doesn’t it tell you?”
• “Which part of this formula stays together as one unit? How do you show two of them?”
• “Give me a compound where the metal test would mislead you.”

Question bank

Recall

1. Name KBr. (Potassium bromide.)
2. What does the prefix penta- mean? (Five.)

Application 3. Name CaO, Li₂S and Al₂O₃. (Calcium oxide, lithium sulfide, aluminium oxide.) 4. Name SO₂ and PCl₃. (Sulfur dioxide, phosphorus trichloride.) 5. Write the formula of magnesium nitrate. (Mg(NO₃)₂.) 6. Name CuCl and CuCl₂. (Copper(I) chloride, copper(II) chloride.)

Explanation 7. A student names Na₂O as “disodium oxide”. Explain the error. (Ionic compounds don’t use prefixes; the charges Na⁺ and O²⁻ already fix the 2:1 ratio. The name is sodium oxide.) 8. Explain why “nitrogen oxide” is not an acceptable name on its own. (Nitrogen and oxygen form several molecular compounds, so the name must state the number of each atom.)

Challenge 9. Name Fe₂(SO₄)₃ and explain how you worked out the Roman numeral. (Iron(III) sulfate: three sulfate ions give 6−, shared by two iron ions, so each is 3+.)

For extra mixed practice with worked answers, send students to naming compounds practice.

Summary

• Teach ionic naming first, with physical ion cards, so students see that charges fix the ratio and prefixes are not needed.
• Use Fe₂O₃ early to break the idea that a Roman numeral counts atoms.
• Introduce prefixes only for molecular compounds, and explain why they are needed there (non-metals combine in several ratios).
• Make “which system?” the first question every time, and include ammonium compounds so the metal test is understood rather than applied blindly.
• Practise in both directions, name to formula and formula to name, in mixed sets.