Write the chemical formulae for: Sodium Carbonate, Aluminum Chloride, and Magnesium Hydroxide.

Chemical Formulae Construction

To write chemical formulae correctly, we use the valencies (combining capacity) of the involved ions and apply the "criss-cross" method.

1. Sodium Carbonate

• Constituent Ions: Sodium (Na⁺) and Carbonate (CO₃^2-)
• Valencies: Sodium = 1, Carbonate = 2
• Method: The 2 from Carbonate goes to Sodium, and the 1 from Sodium goes to Carbonate.
• Formula: Na₂CO₃

2. Aluminum Chloride

• Constituent Ions: Aluminum (Al³⁺) and Chloride (Cl^-)
• Valencies: Aluminum = 3, Chloride = 1
• Method: The 3 from Aluminum goes to Chloride, and the 1 from Chloride goes to Aluminum.
• Formula: AlCl₃

3. Magnesium Hydroxide

• Constituent Ions: Magnesium (Mg²⁺) and Hydroxide (OH^-)
• Valencies: Magnesium = 2, Hydroxide = 1
• Method: The 2 from Magnesium goes to the entire Hydroxide group (using brackets).
• Formula: Mg(OH)₂

Quick Reference Table

Compound Name	Cation	Anion	Chemical Formula
Sodium Carbonate	Na+	CO32-	Na2CO3
Aluminum Chloride	Al3+	Cl-	AlCl3
Magnesium Hydroxide	Mg2+	OH-	Mg(OH)2