How do you calculate oxygen number?

Calculating the oxidation number (also known as oxidation state) of oxygen involves understanding the rules that govern assigning these numbers in chemical compounds. Oxygen usually has an oxidation number of -2, but there are exceptions. Here's a breakdown:

General Rule: -2

In most compounds, the oxidation number of oxygen is -2. This is because oxygen is highly electronegative and tends to gain two electrons to achieve a stable octet.

Exceptions to the -2 Rule

There are key exceptions where oxygen's oxidation number deviates from -2:

• With Fluorine: When oxygen is bonded to fluorine (the most electronegative element), oxygen has a positive oxidation number. For example, in oxygen difluoride (OF₂), oxygen has an oxidation number of +2. This is because fluorine always has an oxidation number of -1.

• In Peroxides: In peroxides, such as hydrogen peroxide (H₂O₂), the oxidation number of oxygen is -1. The general formula for a peroxide is X₂O₂ or XO.

• In Superoxides: In superoxides, such as potassium superoxide (KO₂), the oxidation number of oxygen is -½.

• With Itself (O₂): In elemental form (O₂), the oxidation number of oxygen is 0, as it is a neutral, free element. The same applies to ozone (O₃).

Steps to Calculate Oxygen's Oxidation Number in a Compound

1. Identify Known Oxidation Numbers: Begin by identifying the oxidation numbers of other elements in the compound based on these common rules:
   • Group 1A elements (Li, Na, K, etc.) are almost always +1.
   • Group 2A elements (Be, Mg, Ca, etc.) are almost always +2.
   • Hydrogen is usually +1 (except in metal hydrides, where it's -1).
   • Fluorine is always -1.
2. Use the Overall Charge of the Compound: Remember that the sum of the oxidation numbers in a neutral compound must equal zero. For polyatomic ions, the sum must equal the charge of the ion.
3. Set up an Algebraic Equation: Let 'x' represent the oxidation number of oxygen. Set up an equation based on the known oxidation numbers and the overall charge of the compound or ion.
4. Solve for 'x': Solve the equation for 'x' to determine the oxidation number of oxygen.

Examples

• Water (H₂O):
  • Hydrogen has an oxidation number of +1.
  • The compound is neutral, so the sum of oxidation numbers is 0.
  • Equation: 2(+1) + x = 0
  • x = -2 (Oxygen's oxidation number)
• Oxygen Difluoride (OF₂):
  • Fluorine has an oxidation number of -1.
  • The compound is neutral.
  • Equation: x + 2(-1) = 0
  • x = +2 (Oxygen's oxidation number)
• Hydrogen Peroxide (H₂O₂):
  • Hydrogen has an oxidation number of +1.
  • The compound is neutral.
  • Equation: 2(+1) + 2x = 0
  • 2x = -2
  • x = -1 (Oxygen's oxidation number)

Summary

To accurately determine the oxidation number of oxygen, remember the general rule of -2 and recognize the exceptions, especially when oxygen is bonded to fluorine, is in a peroxide, superoxide, or its elemental form. Always consider the overall charge of the compound or ion and use algebraic equations when needed.