7.3.1 Use of Roman Numerals as Oxidation Number

Roman numerals are used to indicate the oxidation states of elements in compounds, when a metal exhibits variable oxidation states in compounds. For example, in transition metal compounds:

• CuSO₄ is written as copper(II) sulphate.
• FeCl₂ is written as iron(II) chloride.
• FeCl₃ is written as iron(III) chloride.

Simple explanation: When a metal can have more than one oxidation state, Roman numerals tell you which one is present.

Important Points

• - Ionic compounds are electrically neutral — total positive charge = total negative charge.
• - Steps to derive formula:
• 1. Identify ions and their charges.
• 2. Determine ratio to balance charges.
• 3. Write formula with subscripts.
• - Use parentheses for polyatomic ions when more than one is needed.
• - Examples:
• - Calcium chloride: CaCl₂
• - Magnesium oxide: MgO
• - Aluminium sulphate: Al₂(SO₄)₃
• - Criss-cross method: charge on one ion becomes subscript of the other.
• - Roman numerals are used for metals with variable oxidation states (e.g., iron(II), iron(III), copper(II)).

Remember

• - Ionic compounds are neutral.
• - Total positive charge = total negative charge.
• - Ca²⁺ + 2Cl⁻ → CaCl₂
• - Mg²⁺ + O²⁻ → MgO
• - 2Al³⁺ + 3SO₄²⁻ → Al₂(SO₄)₃
• - Use parentheses for polyatomic ions when more than one is needed.
• - Roman numerals for variable oxidation states: iron(II), iron(III), copper(II).

Common Mistakes

• 1. Forgetting that the compound must be neutral. The total positive charge must equal the total negative charge.
• 2. Not using parentheses for polyatomic ions. For example, Al₂(SO₄)₃, not Al₂SO₄₃.
• 3. Forgetting to simplify. Use the smallest whole-number ratio.
• 4. Confusing the charges and subscripts. The charge on one ion becomes the subscript of the other ion (criss-cross).
• 5. Forgetting Roman numerals for variable oxidation states. Iron can be Fe²⁺ or Fe³⁺; Roman numerals distinguish them.
• 6. Writing the anion first. The cation (metal) always comes first in the formula.

Quick Revision

• Formula of an ionic compound: determined by the simplest whole-number ratio of cations to anions that results in a neutral compound.
• Steps:
• Examples:
• Calcium chloride: Ca²⁺ + 2Cl⁻ → CaCl₂
• Magnesium oxide: Mg²⁺ + O²⁻ → MgO
• Aluminium sulphate: 2Al³⁺ + 3SO₄²⁻ → Al₂(SO₄)₃
• Parentheses must be used for polyatomic ions when more than one is needed.
• Roman numerals are used to indicate oxidation states of metals with variable oxidation states (e.g., copper(II) sulphate, iron(II) chloride, iron(III) chloride).

60-Second Revision

• Ionic compounds are neutral.
• Total positive charge = total negative charge.
• CaCl₂: Ca²⁺ + 2Cl⁻.
• MgO: Mg²⁺ + O²⁻.
• Al₂(SO₄)₃: 2Al³⁺ + 3SO₄²⁻.
• Use parentheses for polyatomic ions.
• Warning: don't forget to balance the charges and use the simplest ratio.