Overview
This lesson explains how to determine and write chemical formulas for ionic compounds by understanding ion charges and applying the crisscross method for balancing charges.
Charges of Common Ions
- Group 1 elements (e.g., Li, Na, K) form +1 cations.
- Group 2 elements (e.g., Ca, Mg) form +2 cations.
- Group 13 (3A) elements like Al form +3 cations.
- Group 15 (5A) elements (e.g., N, P) form -3 anions.
- Group 16 (6A) elements (e.g., O, S) form -2 anions.
- Group 17 (halogens: F, Cl, Br, I) form -1 anions.
- Transition metals can have variable charges, often given by Roman numerals.
Writing Formulas with Equal Charge Magnitude
- If cation and anion charges are equal but opposite, combine in a 1:1 ratio.
- Example: NaBr (Na⁺ and Br⁻).
- Example: CaS (Ca²⁺ and S²⁻).
- Example: AlP (Al³⁺ and P³⁻).
Writing Formulas with Unequal Charges (Crisscross Method)
- Swap the magnitude of each ion's charge to the subscript of the other ion (ignore sign).
- Example: Al³⁺ and Cl⁻ → AlCl₃ (1 Al³⁺ and 3 Cl⁻ ions).
- Example: Na⁺ and O²⁻ → Na₂O (2 Na⁺ and 1 O²⁻ ions).
Polyatomic Ions in Formulas
- Use parentheses for multiple polyatomic ions.
- Example: Ba²⁺ and PO₄³⁻ → Ba₃(PO₄)₂.
- Example: Fe³⁺ and SO₄²⁻ → Fe₂(SO₄)₃.
Key Terms & Definitions
- Cation — positively charged ion.
- Anion — negatively charged ion.
- Polyatomic Ion — an ion made of multiple atoms (e.g., SO₄²⁻, PO₄³⁻).
- Crisscross Method — technique of swapping charges as subscripts to balance ionic formulas.
Action Items / Next Steps
- Memorize common ion charges and polyatomic ions.
- Practice writing formulas for ionic compounds using provided methods.