Writing Formulas for Ionic Compounds

Introduction

• Focus on writing formulas for ionic compounds
• Cover basic examples, polyatomic ions, and transition metals

Charges of Common Ions

• Group 1 (Alkali Metals): +1 charge (e.g., Hydrogen, Sodium, Potassium)
• Group 2 (Alkaline Earth Metals): +2 charge (e.g., Magnesium, Calcium)
• Group 3A (Boron Group): +3 charge (e.g., Aluminum)
• Group 5A (Nitrogen Group): -3 charge (e.g., Nitrogen, Phosphorus)
• Group 6A (Chalcogens): -2 charge (e.g., Oxygen, Sulfur)
• Halogens: -1 charge (e.g., Fluoride, Chloride)

Writing Formulas for Ionic Compounds

Equal Charges

• Example: Sodium Chloride (NaCl):

  • Sodium (Na) +1, Chloride (Cl) -1
  • Combine directly as NaCl

• Example: Calcium Sulfide (CaS):

  • Calcium (Ca) +2, Sulfide (S) -2
  • Combine directly as CaS

• Example: Aluminum Nitride (AlN):

  • Aluminum (Al) +3, Nitrogen (N) -3
  • Combine directly as AlN

Different Charges

• Use the crisscross method

  • Charge of one ion becomes the subscript of the other ion

• Example: Lithium Oxide (Li₂O):

  • Lithium (Li) +1, Oxide (O) -2
  • Formula: Li₂O

• Example: Gallium Bromide (GaBr₃):

  • Gallium (Ga) +3, Bromide (Br) -1
  • Formula: GaBr₃

• Example: Magnesium Phosphide (Mg₃P₂):

  • Magnesium (Mg) +2, Phosphide (P) -3
  • Formula: Mg₃P₂

Polyatomic Ions

• Nitrate (NO₃⁻), Nitrite (NO₂⁻): Polyatomic ions
• Phosphate (PO₄³⁻), Phosphite (PO₃³⁻):

Formulas for Common Polyatomic Ions

• Sulfate (SO₄²⁻), Sulfite (SO₃²⁻), Sulfide (S²⁻)
• Perchlorate (ClO₄⁻), Chlorate (ClO₃⁻), Chlorite (ClO₂⁻), Hypochlorite (ClO⁻)

Examples with Polyatomic Ions

• Potassium Sulfate (K₂SO₄):

  • Potassium (K) +1, Sulfate (SO₄) -2
  • Formula: K₂SO₄

• Strontium Phosphate (Sr₃(PO₄)₂):

  • Strontium (Sr) +2, Phosphate (PO₄) -3
  • Formula: Sr₃(PO₄)₂

• Barium Nitrate (Ba(NO₃)₂):

  • Barium (Ba) +2, Nitrate (NO₃) -1
  • Formula: Ba(NO₃)₂

Transition Metals & Variable Charges

• Use Roman numerals to indicate charge

Examples with Transition Metals

• Iron (II) Sulfide (FeS):

  • Iron (Fe) +2, Sulfide (S) -2
  • Formula: FeS

• Iron (III) Sulfide (Fe₂S₃):

  • Iron (Fe) +3, Sulfide (S) -2
  • Formula: Fe₂S₃

• Copper (II) Nitrite (Cu(NO₂)₂):

  • Copper (Cu) +2, Nitrite (NO₂) -1
  • Formula: Cu(NO₂)₂

• Copper (I) Phosphite (Cu₃PO₃):

  • Copper (Cu) +1, Phosphite (PO₃) -3
  • Formula: Cu₃PO₃

• Vanadium (V) Dichromate (V₂(Cr₂O₇)₅):

  • Vanadium (V) +5, Dichromate (Cr₂O₇) -2
  • Formula: V₂(Cr₂O₇)₅

Reducing Subscripts

• Lead (IV) Oxide (PbO₂):
  • Lead (Pb) +4, Oxide (O) -2
  • Formula reduced from Pb₂O₄ to PbO₂

Conclusion

• Use crisscross method and reduce subscripts where applicable
• Familiarize yourself with common ion charges and polyatomic ions
• Understand transition metals' variable charges
• Practice writing formulas regularly