(40) GCSE Chemistry Revision "Formula of Ionic Compounds"

Understanding Ionic Compounds

  • Ionic compounds consist of metal and nonmetal ions.

  • The overall charge of an ionic compound must equal zero, meaning the charges of the ions must cancel each other out.

Key Rule

  • In any ionic compound, the total positive charge must balance the total negative charge.

  • Example compounds must have no overall charge.

Examples of Ionic Compounds

1. Sodium Chloride (NaCl)

  • Ions involved: Sodium ion (Na⁺) and Chloride ion (Cl⁻).

  • Sodium has a +1 charge and Chloride has a -1 charge.

  • Formula: 1 Na⁺ and 1 Cl⁻ combine to give NaCl.

  • Conclusion: NaCl is neutral since +1 and -1 cancel out.

2. Sodium Oxide (Na₂O)

  • Ions involved: Sodium ion (Na⁺) and Oxide ion (O²⁻).

  • Sodium has a +1 charge and Oxide has a -2 charge.

  • Formula requires 2 Na⁺ ions for every 1 O²⁻ ion to balance the charge.

  • Conclusion: Formula is Na₂O.

3. Magnesium Iodide (MgI₂)

  • Ions involved: Magnesium ion (Mg²⁺) and Iodide ion (I⁻).

  • Magnesium has a +2 charge and Iodide has a -1 charge.

  • Formula requires 2 I⁻ ions to cancel out the +2 charge from Mg²⁺.

  • Conclusion: Formula is MgI₂.

4. Lithium Carbonate (Li₂CO₃)

  • Ions involved: Lithium ion (Li⁺) and Carbonate ion (CO₃²⁻).

  • Lithium has a +1 charge and Carbonate has a -2 charge.

  • Formula requires 2 Li⁺ ions to cancel the charge of 1 CO₃²⁻ ion.

  • Conclusion: Formula is Li₂CO₃.

5. Calcium Hydroxide (Ca(OH)₂)

  • Ions involved: Calcium ion (Ca²⁺) and Hydroxide ion (OH⁻).

  • Calcium has a +2 charge and Hydroxide has a -1 charge.

  • Formula requires 2 OH⁻ ions for one Ca²⁺.

  • Important: Use brackets around OH when indicating multiple hydroxide ions to avoid incorrect representation of atoms.

  • Conclusion: Correct formula is Ca(OH)₂.

6. Magnesium Nitrate (Mg(NO₃)₂)

  • Ions involved: Magnesium ion (Mg²⁺) and Nitrate ion (NO₃⁻).

  • Magnesium has a +2 charge and Nitrate has a -1 charge.

  • Requires 2 NO₃⁻ ions to balance the charge of one Mg²⁺ ion.

  • Important: Use brackets around NO₃ when indicating the multiple nitrate ions to avoid confusion about the atom count.

  • Conclusion: Correct formula is Mg(NO₃)₂.

Exam Preparation

  • Expect to identify ionic compounds and balance charges in exam questions.

  • Additional practice and questions available in a revision workbook for further study.