16.3 Writing Chemical formula of ionic compounds using ionic charges (Keynotes, lesson planner and mcqs)

📘 Rule: electrically neutral compound

Potassium permanganate: permanganate ion MnO₄^– + potassium K⁺ → KMnO₄

Ammonium carbonate: NH₄⁺ and CO₃^2- → two ammonium ions needed: (NH₄)₂CO₃

Aluminium sulphate (example 16.5): Al³⁺ + SO₄^2- → cross charges: Al₂(SO₄)₃

📋 Table 16.1 – Common Polyatomic ions

Ion	Name	Ion	Name	Ion	Name
CH₃COO^–	acetate	Cr₂O₇^2-	dichromate	NO₂^–	nitrite
NH₄⁺	ammonium	HCO₃^–	bicarbonate	C₂O₄^2-	oxalate
CO₃^2-	carbonate	HSO₄^–	bisulfate	ClO₄^–	perchlorate
ClO₄^–	chlorate	HSO₃^–	bisulfite	MnO₄^–	permanganate
ClO₃^–	chlorite	OH^–	hydroxide	PO₄^3-	phosphate
CrO₄^2-	chromate	ClO^–	hypochlorite	SO₄^2-	sulfate
CN^–	cyanide	NO₃^–	nitrate	SO₃^2-	sulfite

🧠 Quick memorization & tips

• “Nick the Camel” for -ate vs -ite: chlorate (ClO₃^–) has more oxygen than chlorite (ClO₂^–).
• Permanganate MnO₄^– : deep purple, charge -1 (like permanganate “minus one”).
• Ammonium NH₄⁺ is the only common cation polyatomic; think “ammonia + H”.
• Sulfate SO₄^2- vs sulfite SO₃^2-: “ate” has 4 oxygen, “ite” has 3.
• To write formula: swap charges (crisscross) and reduce if needed. Ex: Ca²⁺ + PO₄^3- → Ca₃(PO₄)₂.

⏳ 35‑min lesson planner

00-05 min ⚡ Hook: KMnO₄ purple power

05-12 min 📖 Direct instruction: KMnO₄, (NH₄)₂CO₃

12-20 min 🧪 Table 16.1 scavenger hunt: match names

20-28 min ✏️ Quick Check! (AlN, Na₂SO₄, NaHCO₃, etc.)

28-35 min ✅ Mini quiz (3 formulas) + exit ticket

📝 16.2 Quick Check!

1. Write formulas using oxidation numbers: Aluminum nitride, Sodium sulfate, Sodium hydrogen carbonate.
AlN, Na₂SO₄, NaHCO₃

2. Write using charges on ions: Ammonium acetate, Calcium phosphate, Potassium chromate.
NH₄CH₃COO, Ca₃(PO₄)₂, K₂CrO₄

🔬 10 MCQs · ionic formulas

Select one answer per question. Correct turns green, wrong ones red after submit.

📌 good guidelines

• Write cation first, then anion. Use parentheses for multiple polyatomic ions.
• Total positive charge must balance total negative charge.
• Memorize common ions from table 16.1: acetate, nitrate, sulfate, carbonate, phosphate.
• Practise criss‑cross method: Mg²⁺ + PO₄^3- → Mg₃(PO₄)₂.
• Always check if subscripts can be reduced (but with polyatomics, keep inside bracket).

⚡ Writing ionic formulas · using charges