Writing Chemical Formulas of Ionic Compounds

Introduction

• Focus on writing the chemical formulas for ionic compounds.
• Key principle: Utilize the charges of ions to determine the formula.

General Steps

1. Identify the Charges: Write the charge of each ion.
2. Same Charges:
   • If ions have the same charge (e.g., +1 and -1), write elements together without subscripts.
3. Different Charges:
   • Use the criss-cross method: exchange charges between ions as subscripts (without carrying the sign).
   • Simplify by omitting subscript of 1.

Examples

Simple Compounds

• Lithium Chloride (LiCl):
  • Lithium: +1, Chloride: -1.
  • Same charges -> LiCl.
• Calcium Sulfide (CaS):
  • Calcium: +2, Sulfide: -2.
  • Same charges -> CaS.
• Aluminum Nitride (AlN):
  • Aluminum: +3, Nitride: -3.
  • Same charges -> AlN.

Different Charges

• Magnesium Bromide (MgBr₂):
  • Magnesium: +2, Bromide: -1.
  • Criss-cross method -> MgBr₂.
• Sodium Sulfide (Na₂S):
  • Sodium: +1, Sulfide: -2.
  • Criss-cross method -> Na₂S.

Practice Problems

• Aluminum Fluoride (AlF₃):
  • Aluminum: +3, Fluoride: -1.
  • Criss-cross method -> AlF₃.
• Calcium Phosphide (Ca₃P₂):
  • Calcium: +2, Phosphide: -3.
  • Criss-cross method -> Ca₃P₂.

Polyatomic Ions

• Aluminum Sulfate (Al₂(SO₄)₃):
  • Aluminum: +3, Sulfate (SO₄): -2.
  • Criss-cross method with parentheses for polyatomic ions -> Al₂(SO₄)₃.
• Strontium Phosphate (Sr₃(PO₄)₂):
  • Strontium: +2, Phosphate (PO₄): -3.
  • Criss-cross method -> Sr₃(PO₄)₂.

Transition Metals and Roman Numerals

• Iron (II) Chloride (FeCl₂):
  • Iron (II): +2, Chloride: -1.
  • Criss-cross method -> FeCl₂.
• Iron (III) Nitrate (Fe(NO₃)₃):
  • Iron (III): +3, Nitrate (NO₃): -1.
  • Criss-cross method -> Fe(NO₃)₃.

Additional Practice

• Copper (I) Phosphate (Cu₃PO₄):
  • Copper (I): +1, Phosphate (PO₄): -3.
  • Criss-cross and simplify -> Cu₃PO₄.
• Copper (II) Phosphide (Cu₃P₂):
  • Copper (II): +2, Phosphide: -3.
  • Criss-cross method -> Cu₃P₂.

Complex Examples

• Tin (IV) Selenide (SnSe₂):
  • Tin (IV): +4, Selenide: -2.
  • Criss-cross method and simplify -> SnSe₂ after reducing Sn₂Se₄.
• Lead (IV) Perbromate (Pb(BrO₄)₄):
  • Lead (IV): +4, Perbromate (BrO₄): -1.
  • Criss-cross and simplify -> Pb(BrO₄)₄.

More Practice

• Vanadium (V) Oxide (V₂O₅):
  • Vanadium (V): +5, Oxide: -2.
  • Criss-cross method -> V₂O₅.

Summary

• Writing formulas involves understanding ion charges and using the criss-cross method.
• Practice is key to mastering the method for different types of ionic compounds.