Overview
This lesson explains how to determine and write the chemical formulas for ionic compounds by understanding ion charges and using the crisscross method for charge balancing.
Ion Charges by Periodic Table Group
- Group 1 elements (e.g., Li, Na, K) form +1 cations.
- Group 2 elements (alkaline earth metals, e.g., Ca, Mg) form +2 cations.
- Group 13 (e.g., Al) forms +3 cations.
- Group 15 (e.g., N, P) forms -3 anions.
- Group 16 (chalcogens, e.g., O, S, Se) form -2 anions.
- Group 17 (halogens, e.g., F, Cl, Br, I) form -1 anions.
- Transition metals can have variable charges, specified by Roman numerals.
Writing Formulas for Ionic Compounds
- List the cation and anion, noting their charges.
- If charges are equal in magnitude, ions combine in a 1:1 ratio (e.g., NaBr, CaS, AlP).
- If charges differ, use the crisscross method: swap the magnitude of the charges to become subscripts for the other ion.
- Example: Al³⁺ and Cl⁻ results in AlCl₃.
- Omit subscript '1' in the final formula.
Using the Crisscross Method
- Example: Na⁺ and O²⁻ → Na₂O (need two Na⁺ to balance one O²⁻).
- Example: Ba²⁺ and PO₄³⁻ → Ba₃(PO₄)₂ (use parentheses for multiple polyatomic ions).
Polyatomic Ions and Parentheses
- When more than one polyatomic ion is used, place the ion in parentheses (e.g., (PO₄)₂).
- Polyatomic ion charges must be memorized (e.g., PO₄³⁻ for phosphate, SO₄²⁻ for sulfate).
Transition Metals and Roman Numerals
- Roman numerals indicate the metal's charge (e.g., iron(III) is Fe³⁺).
- Example: Fe³⁺ and SO₄²⁻ → Fe₂(SO₄)₃.
Key Terms & Definitions
- Cation — positively charged ion.
- Anion — negatively charged ion.
- Polyatomic ion — ion made of multiple atoms acting as a single charged unit.
- Crisscross method — swapping the ion charges to become the subscripts in the formula.
Action Items / Next Steps
- Memorize common ion charges by element group and polyatomic ions.
- Practice writing chemical formulas using the crisscross method.