Overview
This lecture covers how to write formulas for ionic compounds, including those with polyatomic ions and transition metals, focusing on determining charges and applying the crisscross method.
Common Ion Charges
- Group 1 metals (e.g., Na, K, Li) form +1 ions.
- Group 2 metals (e.g., Mg, Ca) form +2 ions.
- Group 3A metals (e.g., Al) form +3 ions.
- Group 5A nonmetals (e.g., N, P) form -3 ions.
- Group 6A nonmetals (e.g., O, S) form -2 ions.
- Halogens (e.g., F, Cl, Br, I) form -1 ions.
Writing Ionic Compound Formulas
- Write the cation and anion with their charges.
- If magnitudes are the same, combine in a 1:1 ratio (e.g., NaCl, CaS, AlN).
- If magnitudes differ, use the crisscross method: swap charges to become subscripts (e.g., Li₂O, GaBr₃, Mg₃P₂).
- Always omit a subscript of 1.
Polyatomic Ions
- Polyatomic ions are charged groups of atoms; common endings: -ate, -ite.
- Use parentheses for multiple polyatomic ions in a formula (e.g., Ba(NO₃)₂).
Example Polyatomic Ions and Formulas
- Nitrate: NO₃⁻, Nitrite: NO₂⁻, Phosphate: PO₄³⁻, Phosphite: PO₃³⁻, Sulfate: SO₄²⁻, Sulfite: SO₃²⁻, Sulfide: S²⁻
- Perchlorate: ClO₄⁻, Chlorate: ClO₃⁻, Chlorite: ClO₂⁻, Hypochlorite: ClO⁻, Chloride: Cl⁻
- Hydroxide: OH⁻, Cyanide: CN⁻, Acetate: C₂H₃O₂⁻, Oxalate: C₂O₄²⁻, Carbonate: CO₃²⁻, Bicarbonate: HCO₃⁻, Chromate: CrO₄²⁻, Dichromate: Cr₂O₇²⁻, Ammonium: NH₄⁺, Permanganate: MnO₄⁻
Transition Metals and Variable Charges
- Transition metals can have multiple charges; use Roman numerals to indicate the charge (e.g., Iron(II), Copper(I)).
- Write formulas using the specified charge, applying the crisscross method.
- Reduce subscripts to the lowest whole number ratio if possible.
Example Applications
- Potassium sulfate: K₂SO₄
- Strontium phosphate: Sr₃(PO₄)₂
- Barium nitrate: Ba(NO₃)₂
- Iron(II) sulfide: FeS
- Iron(III) sulfide: Fe₂S₃
- Copper(II) nitrite: Cu(NO₂)₂
- Copper(I) phosphite: Cu₃PO₃
- Vanadium(V) dichromate: V₂(Cr₂O₇)₅
- Lead(IV) oxide: PbO₂ (reduce subscripts as needed)
Key Terms & Definitions
- Crisscross method — Swap ion charges as subscripts to balance neutral formulas.
- Polyatomic ion — An ion composed of multiple atoms acting as a single charged unit.
- Roman numerals — Indicate the charge of transition metals in compound names.
Action Items / Next Steps
- Memorize charges for common ions and polyatomic ions.
- Practice writing formulas using given ion charges and applying the crisscross method.
- Reduce subscripts to lowest terms in final formulas.