The oxidation number of a free element is always 0. The oxidation number of a monatomic ion equals the charge of the ion. The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements. The oxidation number of O in compounds is usually -2, but it is -1 in peroxides.

How to Find Oxidation State?
Finding the oxidation state of an element in a compound involves a few key steps. Here's a guide to help you determine the oxidation state of an element:

1. Identify the Element
- First, identify the element for which you want to determine the oxidation state in the compound.

2. Determine the Rules
- Understand the common rules for assigning oxidation states. For example, in a neutral compound, the sum of the oxidation states of all the elements must equal zero.

3. Use the Periodic Table
- Use the periodic table to find the usual oxidation states of the element in various compounds. Some elements have fixed oxidation states, while others can have variable oxidation states.

4. Consider the Compound's Overall Charge
- If the compound has an overall charge, the sum of the oxidation states of the elements must equal the charge of the compound.

5. Apply the Rules
- Apply the rules for assigning oxidation states based on the compound's composition and charge. Take into account the electronegativity of the elements involved.

6. Check for Exceptions
- Be aware of any exceptions to the usual rules for assigning oxidation states. Some elements may exhibit unusual oxidation states in certain compounds.

7. Practice and Verify
- Practice assigning oxidation states to different elements in compounds to improve your understanding. Always double-check your calculations to ensure accuracy.
By following these steps and practicing regularly, you can become proficient in determining the oxidation state of an element in a compound.