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Writing Ionic Formulas

Aug 12, 2025

Overview

This lecture explains how to write chemical formulas for ionic compounds from their names by identifying the cation and anion, determining their charges, and applying the correct subscripts. Numerous examples illustrate the process, including rules for polyatomic ions and charge balancing.

Steps for Writing Ionic Compound Formulas

  • Write the symbol/formula for the cation (positive ion) and note its charge; always name and write it first.
  • Write the symbol/formula for the anion (negative ion) and note its charge; always written second.
  • Check if the magnitude of charges is equal; if so, simply combine the ions.
  • If charges are unequal, swap the absolute values of the charges to use as subscripts for the opposite ion.
  • Add parentheses around polyatomic ions before adding a subscript to indicate multiple units of that ion.
  • Do not simplify subscripts inside polyatomic ion formulas (e.g., keep O2 in peroxide).
  • If possible, reduce subscripts to their simplest whole number ratio (e.g., Pb2O4 simplifies to PbO2).

Common Examples

  • Magnesium sulfate: Mg^2+ and SO4^2-; charges cancel → MgSO4.
  • Ammonium sulfide: NH4^+ and S^2-; swap charges → (NH4)2S.
  • Iron(II) nitrate: Fe^2+ and NO3^-; swap charges → Fe(NO3)2.
  • Manganese(IV) chlorite: Mn^4+ and ClO2^-; swap charges → Mn(ClO2)4.
  • Cesium dihydrogen phosphate: Cs^+ and H2PO4^-; charges cancel → CsH2PO4.
  • Lead(IV) oxide: Pb^4+ and O^2-; swap charges and simplify → PbO2.
  • Sodium peroxide: Na^+ and O2^2-; swap charges → Na2O2 (keep O2 as is).
  • Silver dichromate: Ag^+ and Cr2O7^2-; swap charges → Ag2Cr2O7.
  • Barium phosphate: Ba^2+ and PO4^3-; swap charges → Ba3(PO4)2.
  • Nickel(II) hydroxide: Ni^2+ and OH^-; swap charges → Ni(OH)2.
  • Lead(IV) carbonate: Pb^4+ and CO3^2-; swap, then simplify → Pb(CO3)2.
  • Calcium oxalate: Ca^2+ and C2O4^2-; charges cancel → CaC2O4.
  • Magnesium acetate: Mg^2+ and C2H3O2^-; swap charges → Mg(C2H3O2)2.
  • Cobalt(III) cyanide: Co^3+ and CN^-; swap charges → Co(CN)3.
  • Zinc sulfite: Zn^2+ and SO3^2-; charges cancel → ZnSO3.
  • Aluminum periodate: Al^3+ and IO4^-; swap charges → Al(IO4)3.

Key Terms & Definitions

  • Cation — positively charged ion, named and written first in the formula.
  • Anion — negatively charged ion, named and written second in the formula.
  • Polyatomic Ion — an ion composed of multiple atoms covalently bonded, acting as a single unit (e.g., SO4^2-, NO3^-).
  • Subscript — number written below and to the right of an element or group, indicating how many units are present.
  • Charge Swapping — using the magnitude of one ion’s charge as the subscript for the other ion.

Action Items / Next Steps

  • Practice more examples from the workbook or using an ion chart.
  • Memorize common polyatomic ions, their formulas, and charges.
  • Review simplifying subscripts and rules for parentheses with polyatomic ions.

Full transcript