CHEM104 Lecture Chapter 3 Overview C: Writing Formulas for Ionic Compounds

Introduction

• Focus on writing formulas for ionic compounds given the compound's name.
• Emphasis on balancing charges to write a correct ionic compound formula.

Steps for Writing Formulas

1. Identify Ions and Their Charges

   • Write each ion with its charge.
   • Use the lowest common multiple (LCM) to balance charges.

2. Balancing Charges

   • Total positive charge must equal total negative charge.
   • Charges are not included in the final formula.
   • Use subscripts to indicate the number of each ion.

Examples

• Potassium Chloride (KCl):

  • Potassium ion: +1, Chloride ion: -1
  • 1:1 ratio, formula is KCl.

• Silver Sulfide (Ag2S):

  • Silver ion: +1, Sulfide ion: -2
  • Need 2 silver ions for every 1 sulfide ion.

• Calcium Oxide (CaO):

  • Calcium ion: +2, Oxide ion: -2
  • 1:1 ratio, formula is CaO.

• Iron(II) Chloride (FeCl2):

  • Iron ion: +2, Chloride ion: -1
  • Need 2 chloride ions for every 1 iron ion.

• Aluminum Oxide (Al2O3):

  • Aluminum ion: +3, Oxide ion: -2
  • LCM of 3 and 2 is 6.
  • Need 2 aluminum ions and 3 oxide ions.

• Chromium(VI) Oxide (CrO3):

  • Chromium ion: +6, Oxide ion: -2
  • Need 3 oxide ions for each chromium ion.

Polyatomic Ions

• Subscripts:

  • Must include the subscript part of the formula.
  • Example: NH4 for ammonium, not NH.

• Common Polyatomic Ions:

  • Acetate: CH3CO2 or C2H3O2
  • Cyanide: CN
  • Bicarbonate: HCO3

• "Ate" vs "Ite":

  • "Ite" ions have one less oxygen than "Ate" ions.
  • Examples: Nitrate (NO3) vs. Nitrite (NO2), Sulfate (SO4) vs Sulfite (SO3).

• Parentheses Usage:

  • Use parentheses if more than one polyatomic ion is needed.
  • Example: Cu(CN)2 for copper(II) cyanide.

Additional Examples

• Lithium Hydroxide (LiOH):

  • Lithium: +1, Hydroxide: -1
  • Formula is LiOH.

• Zinc Nitrite (Zn(NO2)2):

  • Zinc: +2, Nitrite: -1
  • Need 2 nitrite ions.

• Barium Carbonate (BaCO3):

  • Barium: +2, Carbonate: -2
  • 1:1 ratio, formula is BaCO3.

• Copper(II) Cyanide (Cu(CN)2):

  • Copper ion: +2, Cyanide ion: -1
  • Need 2 cyanide ions.

• Iron(II) Phosphate (Fe3(PO4)2):

  • Iron ion: +2, Phosphate ion: -3
  • LCM of 2 and 3 is 6.
  • Need 3 iron ions and 2 phosphate ions.

• Gold(III) Bicarbonate (Au(HCO3)3):

  • Gold: +3, Bicarbonate: -1
  • Need 3 bicarbonate ions.

Conclusion

• Accurate writing of ionic formulas requires balancing charges and proper use of parentheses and subscripts.
• Pay attention to ion suffixes like "ate" and "ite" as they indicate different compositions.