How to Write Ionic Formulas

Prerequisite Skills

• Drawing Atoms: Be comfortable drawing atoms, such as sodium and fluorine.
• Converting Atoms to Ions:
  • Sodium atom has one electron in the outermost level and will lose it.
  • Fluorine atom has 7 electrons and will gain one to complete its outer shell.
  • Sodium becomes a sodium ion when it loses an electron.

Understanding Ions

• Electrons in Ionic Bonds:
  • Sodium loses an electron, which is gained by fluorine.
  • Positive ions are termed "cations" and negative ions are "anions".

Table of Ions

• Ionic Symbols: Important for the first 20 elements.
• Cations vs Anions:
  • Cations have the same names as their atoms.
  • Anions are named differently, e.g., "fluoride" for fluorine ion.
  • Familiarize with names, especially for NCEA Level One.
  • Example: Fe2+ is "Iron 2", Fe3+ is "Iron 3".
• Ammonium Ion: Different from ammonia.

Writing Ionic Formulas

Example 1: Silver Chloride

• Finding Charges: Look up silver and chloride in the table.
• Charge Comparison:
  • Silver: +1 charge.
  • Chloride: -1 charge.
• Formulation Rule: Equal and opposite charges are written together, e.g., AgCl.
• Note: Charges are not shown in the final formula.

Example 2: Barium Sulfate

• Charge Comparison:
  • Barium: +2 charge.
  • Sulfate: -2 charge.
• Formulation: Write together as BaSO4 without showing charges.

Rule 2: Using Multiples

• Example: Calcium Chloride
  • Calcium: +2 charge.
  • Chloride: -1 charge.
  • Balanced Formula: CACl₂ (two chlorides needed to balance charge).
• Example: Silver Sulfate
  • Silver: +1 charge.
  • Sulfate: -2 charge.
  • Balanced Formula: Ag₂SO₄ (two silvers needed).

Rule 3: Using Brackets for Compound Ions

• Compound Ions: Ions with more than one type of element.
• Example: Magnesium Nitrate
  • Magnesium: +2 charge.
  • Nitrate: -1 charge.
  • Balanced Formula: Mg(NO₃)₂ (brackets used around nitrate).

Conclusion

• Review Rules:
  • Write the ions together when charges are equal and opposite.
  • Use multiples to balance charges when necessary.
  • Use brackets for compound ions when multiples are needed.
• Practice: Familiarize with the table of ions and the naming conventions to improve accuracy in writing ionic formulas.