Writing Chemical Formulas of Ionic Compounds

Overview

• Understanding the charges of ions is crucial for writing chemical formulas of ionic compounds.
• Ions are formed based on the elements' group in the periodic table.

Ion Charges by Group

Group 1: Alkali Metals

• Elements: Lithium, Sodium, Potassium
• Charge: +1 (Cations)

Group 2: Alkaline Earth Metals

• Elements: Calcium, Magnesium
• Charge: +2 (Cations)

Transition Metals

• Variable charges
• Not the main focus for basic ionic compound formulas

Group 13 (3A)

• Element: Aluminum
• Charge: +3 (Cation)

Group 14 (4A)

• Elements: Carbon, Silicon, Germanium
• Rarely form ionic compounds

Group 15 (5A)

• Elements: Nitrogen, Phosphorus
• Charge: -3 (Anions)

Group 16: Chalcogens

• Elements: Oxygen, Sulfur, Selenium
• Charge: -2 (Anions)

Group 17: Halogens

• Elements: Fluoride, Chloride, Bromide, Iodide
• Charge: -1 (Anions)

Writing Chemical Formulas

When Charges are Equal in Magnitude

1. Example: Sodium Bromide (NaBr)

   • Sodium (Na): +1
   • Bromide (Br): -1
   • Formula: NaBr

2. Example: Calcium Sulfide (CaS)

   • Calcium (Ca): +2
   • Sulfide (S): -2
   • Formula: CaS

3. Example: Aluminum Phosphide (AlP)

   • Aluminum (Al): +3
   • Phosphorus (P): -3
   • Formula: AlP

When Charges Differ

1. Strategy: Crisscross Method

   • Swap the charges to subscripts.

2. Example: Aluminum Chloride (AlCl₃)

   • Aluminum (Al): +3
   • Chloride (Cl): -1
   • Formula: AlCl₃

3. Example: Sodium Oxide (Na₂O)

   • Sodium (Na): +1
   • Oxide (O): -2
   • Formula: Na₂O

Polyatomic Ions

1. Example: Barium Phosphate (Ba₃(PO₄)₂)
   • Barium (Ba): +2
   • Phosphate (PO₄): -3
   • Remember to use parentheses for multiple polyatomic ions.

Transition Metals

1. Example: Iron (III) Sulfate (Fe₂(SO₄)₃)
   • Iron (Fe): +3 (Roman numeral indicates charge)
   • Sulfate (SO₄): -2
   • Formula: Fe₂(SO₄)₃

Tips

• Memorize common ion charges and polyatomic ions.
• Use the crisscross method to balance charges in the formula.

External Resources

• Check online resources for videos on polyatomic ions for further understanding.

Conclusion

• Understanding the concept of ion charges and the crisscross method is essential for writing chemical formulas of ionic compounds effectively.