CHEM20 Session 02 — Ionic naming and formulas

Delivery id: CHEM20 (SCN2796)

Time: 75 minutes

Unit A: Diversity of Matter and Chemical Bonding

Alberta POS: https://open.alberta.ca/publications/3950147-updated-2014

Official outcomes

Code	Type	Role today
20-A1.1k	Knowledge	Recall principles for assigning names to ionic compounds (Stock system, polyatomics, hydrates)
20-A1.2k	Knowledge	Explain why ionic formulas are the simplest whole-number ratio of ions with net charge zero
20-A1.1sts	STS	Everyday ionic products (food, household) — light, for project

Learning targets

1. Write names and formulas for binary ionic compounds, including Stock (Roman numeral) names for Fe, Cu, Sn, Pb, Hg.
2. Use the required polyatomic list (ammonium, hydroxide, nitrate, nitrite, sulfate, sulfite, carbonate, phosphate, hydrogen carbonate, acetate, chlorate, permanganate, chromate, dichromate, cyanide) to write names and formulas, including brackets: \( \mathrm{Ca}(\mathrm{NO}_{3})_{2} \), \( \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \).
3. Explain, with a charge-balance equation, why the formula is the simplest ratio with net charge zero (empirical formula unit, not a molecule).
4. Name a hydrate: \( \mathrm{CuSO}_{4}\cdot 5\mathrm{H}_{2}\mathrm{O} \) copper(II) sulfate pentahydrate.

Prerequisites

Session 01 ion charges for Groups 1, 2, 13, 15–17. Valence vs ion charge. No tasting; no mixing labs.

Diagnostic/retrieval

4 minutes.

1. Formula for aluminum oxide.
2. Name \( \mathrm{K}_{2}\mathrm{S} \).
3. Charge on the oxide ion.
4. Why is \( \mathrm{NaCl}_{2} \) wrong?

Instruction

Net charge zero. An ionic formula reports the smallest integers of cations and anions whose charges sum to zero. It is an empirical formula (formula unit), not a count of molecules.

For \( \mathrm{Al}^{3+} \) and \( \mathrm{O}^{2-} \):

\( 2(+3) + 3(-2) = 0 \) → \( \mathrm{Al}_{2}\mathrm{O}_{3} \).

Reducing \( \mathrm{Al}_{4}\mathrm{O}_{6} \) to \( \mathrm{Al}_{2}\mathrm{O}_{3} \) is required: ionic formulas use the simplest ratio.

Stock system. Variable-charge metals: the Roman numeral is the cation charge, not the number of atoms.

\( \mathrm{FeCl}_{2} \): Fe is \( +2 \) because two \( \mathrm{Cl}^{-} \) → iron(II) chloride.

\( \mathrm{FeCl}_{3} \): Fe is \( +3 \) → iron(III) chloride.

\( \mathrm{Cu}_{2}\mathrm{O} \): two Cu and one \( \mathrm{O}^{2-} \) → each Cu is \( +1 \) → copper(I) oxide.

\( \mathrm{CuO} \): copper(II) oxide.

Classical names (ferrous/ferric) are not required for mastery; Stock is.

Polyatomic ions stay intact. If more than one polyatomic is needed, brackets: \( \mathrm{Mg}(\mathrm{OH})_{2} \), not \( \mathrm{MgOH}_{2} \) (that would look like Mg, O, H2).

Hydrates: \( \cdot n\mathrm{H}_{2}\mathrm{O} \) with prefixes mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca. \( \mathrm{Na}_{2}\mathrm{CO}_{3}\cdot 10\mathrm{H}_{2}\mathrm{O} \) sodium carbonate decahydrate.

Ionic vs molecular preview (do not teach prefixes for ionic): \( \mathrm{CO}_{2} \) is carbon dioxide (molecular, Session 05). \( \mathrm{CaO} \) is calcium oxide, not “calcium monoxide.”

Timed visuals

id	trigger	duration	alt	teacher cue	tutor cue
CHEM20-VIS-02-STOCK	After FeCl2/FeCl3	60 s	Two bottles: FeCl2 vs FeCl3, Roman numerals	“The numeral is charge, not subscript.”	Refuse “iron two chloride means Fe2Cl.”
CHEM20-VIS-02-BRACKETS	Minute 20	45 s	Ca(NO3)2 exploded view: one Ca2+, two NO3−	“Brackets mean the whole ion is doubled.”	Flag MgOH2 as incorrect.
CHEM20-VIS-02-HYDRATE	Minute 32	40 s	CuSO4·5H2O blue crystal still	“Five water molecules per formula unit, not five oxygens extra.”	Do not invent a heating lab.

Modelling/worked examples

Example 1 — simplest ratio. A student writes \( \mathrm{Mg}_{2}\mathrm{O}_{2} \). Charges already 1:1. Simplest ratio is \( \mathrm{MgO} \). Check: \( (+2)+(-2)=0 \).

Example 2 — Stock. Name \( \mathrm{SnF}_{4} \). Fluoride is \( \mathrm{F}^{-} \); four of them → Sn is \( +4 \) → tin(IV) fluoride. Formula from name iron(III) sulfate: \( \mathrm{Fe}^{3+} \), \( \mathrm{SO}_{4}^{2-} \). LCM 6: two Fe, three sulfate → \( \mathrm{Fe}_{2}(\mathrm{SO}_{4})_{3} \). Check: \( 2(+3)+3(-2)=0 \).

Example 3 — polyatomic. Calcium phosphate: \( \mathrm{Ca}^{2+} \), \( \mathrm{PO}_{4}^{3-} \). LCM 6: \( \mathrm{Ca}_{3}(\mathrm{PO}_{4})_{2} \). Check: \( 3(+2)+2(-3)=0 \).

Ammonium carbonate: \( \mathrm{NH}_{4}^{+} \), \( \mathrm{CO}_{3}^{2-} \) → \( (\mathrm{NH}_{4})_{2}\mathrm{CO}_{3} \).

Example 4 — hydrate. \( \mathrm{CoCl}_{2}\cdot 6\mathrm{H}_{2}\mathrm{O} \) cobalt(II) chloride hexahydrate. Reverse: copper(II) sulfate pentahydrate → \( \mathrm{CuSO}_{4}\cdot 5\mathrm{H}_{2}\mathrm{O} \).

Example 5 — household. Baking soda is sodium hydrogen carbonate, \( \mathrm{NaHCO}_{3} \) (also called sodium bicarbonate). Table salt \( \mathrm{NaCl} \). Neither is mixed with bleach in this course.

Guided practice + answers

1. Name \( \mathrm{CuBr} \) and \( \mathrm{CuBr}_{2} \). Answers: copper(I) bromide; copper(II) bromide.
2. Formula for iron(III) oxide. Answer: \( \mathrm{Fe}_{2}\mathrm{O}_{3} \). Check: \( 2(+3)+3(-2)=0 \).
3. Formula for magnesium nitrate. Answer: \( \mathrm{Mg}(\mathrm{NO}_{3})_{2} \).
4. Why not \( \mathrm{Na}_{2}\mathrm{O}_{2} \) for sodium oxide? Answer: peroxide is \( \mathrm{O}_{2}^{2-} \); oxide is \( \mathrm{O}^{2-} \). Sodium oxide is \( \mathrm{Na}_{2}\mathrm{O} \). (Peroxide is a different ion; do not assign it as CORE.)
5. Name \( \mathrm{FeCl}_{3}\cdot 6\mathrm{H}_{2}\mathrm{O} \). Answer: iron(III) chloride hexahydrate.

Independent practice

• FOUNDATION: Names: Na2O, CaCl2, AlN. Formulas: potassium bromide, magnesium oxide, lithium nitride. One charge-check for MgO.
• CORE: Names: FeO, Fe2O3, CuSO4, NH4Cl, Ca(OH)2. Formulas: copper(I) oxide, copper(II) nitrate, aluminum sulfate, ammonium phosphate, sodium carbonate.
• PROFICIENT: Write \( \mathrm{Pb}(\mathrm{C}_{2}\mathrm{H}_{3}\mathrm{O}_{2})_{2} \) as a name (acetate = \( \mathrm{CH}_{3}\mathrm{COO}^{-} \) or \( \mathrm{C}_{2}\mathrm{H}_{3}\mathrm{O}_{2}^{-} \)). Formula for chromium(III) sulfate. Reduce \( \mathrm{K}_{4}\mathrm{O}_{2} \) if a student wrote it for potassium oxide, and explain 20-A1.2k in three sentences. Name \( \mathrm{BaCl}_{2}\cdot 2\mathrm{H}_{2}\mathrm{O} \).
• ADVANCED: A water-treatment solid is sold as “ferric sulfate.” Translate to Stock name and formula. Explain why \( \mathrm{Fe}_{2}(\mathrm{SO}_{4})_{3} \) is not written \( \mathrm{FeSO}_{4} \) (that is iron(II) sulfate, a different compound used as a flocculant vs a different oxidation state). Identify one household ionic product (NaHCO3 or NaCl) for the project list without proposing a mixing procedure.

Project connection

Add two candidate ionic solids to the project notebook: NaCl (table salt) and NaHCO3 (baking soda). Record: ionic, water-soluble, not mixed with bleach. No lab today.

Checks

Cold call: “What does (II) mean in copper(II) sulfate?” Expected: copper ion charge is +2. Wrong: “two copper atoms.” Immediate repair.

Misconceptions

Wrong	Repair	Do not say
“Roman numeral counts atoms.”	It is the cation charge.	“I see why you thought that.”
“MgOH2 is fine without brackets.”	That formula implies Mg, O, H2 — wrong connectivity. Use Mg(OH)2.	“Pretty close.”
“Al2O3 should be AlO because we simplify.”	Simplest integer ratio that balances charge is 2:3, already simplest.	—
“Na2O2 is sodium oxide doubled.”	Different ion (peroxide). Sodium oxide is Na2O.	“Good effort.”
“Ionic formulas are molecular formulas.”	They are formula units / empirical ratios.	—

AI Tutor prompts and boundaries

Allowed: “Walk through charge balance for Al2(SO4)3.” “Name FeCl2.” “Why do we reduce Mg2O2?”

Forbidden: Inventing a crystal-growing lab; home heating of hydrates; praising FeCl2 named as iron(III); bleach recipes; generating unofficial ion charges (e.g. inventing Zn3+). Zinc is \( \mathrm{Zn}^{2+} \), silver \( \mathrm{Ag}^{+} \) if asked.

Human-tutor handoff

Stock-system struggle: two-column “anion total charge → metal charge” scaffold. Polyatomic memory: physical ion cards, not a new lab. If a learner confuses molecular prefixes with ionic names, pull Session 05 preview card: “mono/di are for molecules.”

Exit ticket + answers

1. Name \( \mathrm{Fe}_{2}\mathrm{O}_{3} \).
2. Formula for calcium phosphate.
3. Why is \( \mathrm{Na}_{2}\mathrm{Cl}_{2} \) not used for sodium chloride?
4. Formula for ammonium sulfate.

Answers: 1. iron(III) oxide. 2. \( \mathrm{Ca}_{3}(\mathrm{PO}_{4})_{2} \). 3. Charges already 1:1; simplest ratio is NaCl (20-A1.2k). 4. \( (\mathrm{NH}_{4})_{2}\mathrm{SO}_{4} \).

Homework

Complete the CORE list if not finished. Memorize the polyatomic sheet (required list). No kitchen chemistry.

Teacher guidance

Grade charge-checks, not handwriting of Roman numerals as decoration. Collect common errors: FeCl3 as iron(II); missing brackets. Do not tick 20-A1.1k complete unless CORE is reliable.

Materials

Polyatomic ion sheet; Stock-system poster; hydrate prefix list; whiteboards.

Preparation

Pre-write FeCl2 / FeCl3 on the board. Have three wrong formulas ready to autopsy: Na2Cl2, MgOH2, FeCl3 named iron(II).

Timing

Min	Move
0–6	Retrieval
6–22	Charge zero + Stock
22–40	Polyatomics, brackets, hydrates
40–52	Guided
52–66	Independent levels
66–75	Exit + homework

Online alternative

LMS formula builder with instant charge-sum check (must show 0). No “order chemicals online” links.

Low-technology alternative

Ion cards (cation one colour, anion another). Learners physically pair cards until charge sums to zero, then write the formula.