Chemistry Solution: Assigning Oxidation Numbers

Introduction

• Oxidation numbers are a bookkeeping system for tracking electrons in compounds.
• They are not actual charges on the atoms.

Rules for Assigning Oxidation Numbers

1. Lone Element Rule: The oxidation number for any lone element is 0.
   • Examples: Potassium (K), Aluminum (Al), O2, S8.
2. Electronegativity Priority: Assign oxidation numbers starting with the most electronegative elements:
   • Fluorine: Always -1.
   • Oxygen: Usually -2, except when bonded to fluorine.
3. Group Number Reference:
   • Group 1A: +1
   • Group 2A: +2
   • Aluminum: +3
4. Hydrogen: +1 when with nonmetals, -1 when with metals.
5. Overall Charge: The sum of oxidation numbers must equal the overall charge on the compound.

Examples for Assigning Oxidation Numbers

Example 1: OF2

• Fluorine: -1 (x2) = -2 (total)
• Oxygen: +2 (to balance, since total must equal 0)
• Result: Fluorine = -1, Oxygen = +2

Example 2: Fe2O3

• Oxygen: -2 (x3) = -6 (total)
• Total charge of compound = 0 → Iron must be +6.
• Iron oxidation state: +3 (since there are 2 iron atoms, +3 x 2 = +6)
• Result: Iron = +3, Oxygen = -2

Example 3: CH4

• Hydrogen: +1 (x4) = +4 (total)
• Total charge = 0 → Carbon must be -4.
• Result: Hydrogen = +1, Carbon = -4

Example 4: NaHCO3

• Oxygen: -2 (x3) = -6 (total)
• Sodium: +1
• Hydrogen: +1
• Total oxidation state for Carbon must be +4 to balance out to 0.
• Result: Oxygen = -2, Sodium = +1, Hydrogen = +1, Carbon = +4

Conclusion

• Remember the key rules and practice assigning oxidation numbers to become proficient in identifying oxidation states in various compounds.