Overview
This lecture explains how to write chemical formulas for ionic compounds by using ion charges and applying the crisscross method for balancing charges.
Writing Formulas for Ionic Compounds
- Write the charge of each ion involved in the compound.
- If the charges are equal and opposite (e.g., +1 and -1, +2 and -2), write the symbols together with no subscripts.
- For different charges, use the crisscross method: make the magnitude of each ion’s charge the subscript of the other ion.
Examples with Simple Ions
- Lithium chloride: Li⁺ and Cl⁻ → LiCl.
- Calcium sulfide: Ca²⁺ and S²⁻ → CaS.
- Aluminum nitride: Al³⁺ and N³⁻ → AlN.
- Magnesium bromide: Mg²⁺ and Br⁻ → MgBr₂.
- Sodium sulfide: Na⁺ and S²⁻ → Na₂S.
Examples with Polyatomic Ions
- Aluminum sulfate: Al³⁺ and SO₄²⁻ → Al₂(SO₄)₃ (parentheses used for multiple polyatomic ions).
- Strontium phosphate: Sr²⁺ and PO₄³⁻ → Sr₃(PO₄)₂.
Transition Metals and Roman Numerals
- Iron(II) chloride: Fe²⁺ and Cl⁻ → FeCl₂.
- Iron(III) nitrate: Fe³⁺ and NO₃⁻ → Fe(NO₃)₃.
- Copper(I) phosphate: Cu⁺ and PO₄³⁻ → Cu₃PO₄.
- Copper(II) phosphide: Cu²⁺ and P³⁻ → Cu₃P₂.
Advanced Examples and Reducing Subscripts
- Tin(IV) selenide: Sn⁴⁺ and Se²⁻ → Sn₂Se₄, reduced to SnSe₂.
- Lead(IV) perbromate: Pb⁴⁺ and BrO₄⁻ → Pb(BrO₄)₄.
- Vanadium(V) oxide: V⁵⁺ and O²⁻ → V₂O₅.
Key Terms & Definitions
- Ionic compound — Compound formed from positive and negative ions.
- Crisscross method — Technique for using ion charges as subscripts to balance formulas.
- Polyatomic ion — Ion composed of multiple atoms with a net charge.
- Roman numeral — Indicates the charge of a transition metal ion in a compound.
Action Items / Next Steps
- Practice writing formulas for given ionic compounds, especially those with polyatomic ions or transition metals.
- Review periodic table group trends for common ion charges.