Lecture Notes: Oxidation Numbers and How to Find Them

Key Concepts

1. Oxidation States of Pure Elements

   • The oxidation state of any pure element is always 0.
   • Examples: Oxygen gas (O₂), Fluorine gas (F₂), Phosphorus (P) are all 0.

2. Oxidation States of Ions

   • Monatomic ions have oxidation states equal to their charge.
     • Example: Zn²⁺ has an oxidation state of +2.
   • Diatomic ions, like mercury (Hg₂²⁺), require division to find individual oxidation states.
     • Example: Each Hg in Hg₂²⁺ has an oxidation state of +1.

3. Special Ions

   • Peroxide ion (O₂²⁻): Each oxygen has an oxidation state of -1.
   • Superoxide ion (O₂⁻): Each oxygen has an oxidation state of -0.5.

Oxidation States in Compounds

1. Fluorine

   • Always has an oxidation state of -1 when in compounds.

2. Oxygen

   • Typically -2, except in peroxides (-1), superoxides (-0.5), or when bonded to fluorine.

3. Hydrogen

   • +1 when bonded to nonmetals.
   • -1 when bonded to metals.

Determining Oxidation States

1. Example 1: MgCl₂

   • Cl is -1, hence Mg must be +2.

2. Example 2: AlF₃

   • F is -1, hence Al must be +3.

3. Example 3: V₂O₅

   • O is -2, hence V must be +5.

Polyatomic Ions

1. Sulfate (SO₄²⁻):

   • O is -2, S is +6.

2. Phosphate (PO₄³⁻):

   • O is -2, P is +5.

3. Nitrate (NO₃⁻):

   • O is -2, N is +5.

4. Perchlorate (ClO₄⁻):

   • O is -2, Cl is +7.

Electronegativity and Oxidation Numbers

• Electronegativity trends: increases towards fluorine on the periodic table.
  • Example values: H (2.1), B (2.0), C (2.5), N (3.0), O (3.5), F (4.0)
• More electronegative elements tend to acquire negative charges.
• In OF₂, oxygen is +2 because F is more electronegative.

Examples with Explanation

• HCl:
  • Cl is -1, H is +1.
• NaH:
  • Na is +1, H is -1.
• BH₃:
  • H is -1, B is +3.
• SO₂:
  • O is -2, S is +4.
• NH₃:
  • N is -3, H is +1.
• NO₂:
  • O is -2, N is +4.

Oxidation States in Various Compounds

• CH₄:
  • C is -4 because it is more electronegative than H.
• CO₂:
  • C is +4 because O is more electronegative.
• C₃H₈:
  • Average oxidation state of C is -8/3.
• Fe₃O₄:
  • Average oxidation state of Fe is +8/3.

Polyatomic Ions with Multiple Elements

• HSO₃⁻:
  • O is -2, H is +1, S is +4.
• K₂CrO₄:
  • O is -2, K is +1, Cr is +6.
• KHCO₃:
  • O is -2, H is +1, K is +1, C is +4.

Non-uniform Oxidation States

• BrCl₃:
  • Cl is -1, Br is +3.
• IBr₅:
  • Br is -1, I is +5.

Conclusion

• Understanding the concept of electronegativity and its impact on oxidation states is critical for solving problems involving oxidation numbers.