Binary Ionic Compounds: Nomenclature & Formula Writing (Key Points)

Basics of Binary Ionic Compounds

• Ionic compounds contain positive and negative ions; overall neutral.
• Binary ionic compound: monatomic cation + monatomic anion.
• Monatomic anions end in -ide (e.g., sulfide, phosphide).

Writing Chemical Formulas

• Steps:
  • Identify the cation and anion and their charges.
  • Combine ions in the smallest whole-number ratio so total positive charges equal total negative charges.
  • Write the chemical formula reflecting this ratio.
• Criss-Cross Method:
  • A convenient way: the cation charge becomes the subscript of the anion; the anion charge becomes the subscript of the cation.
  • Example: potassium sulfide: $K_2S$.
  • Ensure the ratio is the smallest whole-number ratio (no common factor > 1).

Naming Binary Ionic Compounds

• Name cation first; if monatomic, use the element name.
• Name the anion second; if monatomic, change ending to -ide.
• For cations with multiple charges, add roman numeral indicating the charge: e.g., iron(II) chloride.

Cations with Multiple Charges

• Some transition metals form different ions (e.g., Pb^{2+}, Pb^{4+}).
• With chlorine:
  • PbCl_2 corresponds to Pb^{2+} (lead(II) chloride).
  • PbCl_4 corresponds to Pb^{4+} (lead(IV) chloride).
• Not all transition metals form multiple cations.
• Roman numerals indicate the charge in the formula.

Fixed-Charge Cations

• Some cations have fixed charges (e.g., group 1: +1; group 2: +2; Ca^{2+}; K^{+}).
• They do not require roman numerals when naming formulas.

Examples

• Potassium sulfide: $K_2S$
• Lead(II) chloride: $PbCl_2$
• Lead(IV) chloride: $PbCl_4$