Overview
This lecture covers the naming and formula-writing rules for compounds containing polyatomic ions, focusing on endings, charge conventions, and the crisscross method.
Polyatomic Ion Endings & Charges
- Most polyatomic ions end with “-ate” (more oxygens) or “-ite” (fewer oxygens).
- Only hydroxide (OH⁻) and cyanide (CN⁻) have the “-ide” ending among polyatomics.
- Nearly all polyatomic ions have negative charges; ammonium (NH₄⁺) is the only common positive one.
Naming & Formulas with Polyatomic Ions
- The “-ate” ending indicates a higher number of oxygens than the “-ite” form.
- Prefix “bi-” or “hydrogen” indicates presence of H⁺ (e.g., bicarbonate/hydrogen carbonate, HCO₃⁻).
- Use parentheses around a polyatomic ion in a formula when more than one is present (e.g., Ca(OH)₂).
- Do not use parentheses for monoatomic ions, even if more than one is present (e.g., Li₃PO₄, not (Li)₃PO₄).
- Do not use parentheses for a single polyatomic ion (e.g., NaOH).
Crisscross Method for Compound Formation
- Use the crisscross method to balance charges in ionic compounds.
- The charge on one ion becomes the subscript on the other ion.
Distinguishing Ions by Name Endings
- “-ite” and “-ate” endings indicate polyatomic ions.
- “-ide” generally indicates monoatomic anions found on the right side of the periodic table, except for hydroxide and cyanide.
Steps for Naming Compounds
- Check if the compound contains a metal and a non-metal to determine if it's ionic.
- Identify if the metal is a main group element (charges from periodic table) or a transition metal (may need Roman numerals).
- Determine if the non-metal is a monoatomic ion (“-ide”) or a polyatomic ion (“-ate”, “-ite”, sometimes “-ide”).
Examples
- Cupric acetate: Copper(II) ion (Cu²⁺) and acetate (C₂H₃O₂⁻), formula is Cu(C₂H₃O₂)₂.
- Ferric hypochlorite: Iron(III) ion (Fe³⁺) and hypochlorite (ClO⁻), adjust using crisscross method.
- Ammonium dichromate: Combines two polyatomic ions (NH₄⁺ and Cr₂O₇²⁻); formula is (NH₄)₂Cr₂O₇.
Key Terms & Definitions
- Polyatomic ion — An ion made of two or more atoms covalently bonded, acting as a single charged unit.
- Crisscross method — Technique for determining the subscripts in ionic compound formulas to balance charges.
- Monoatomic ion — An ion consisting of only one atom.
- Parentheses — Used in chemical formulas when more than one of a polyatomic ion is present.
Action Items / Next Steps
- Memorize the list of common polyatomic ions for the next quiz.
- Practice writing formulas and naming compounds using the crisscross method.