Oxidation number is the charge which an atom of an element has in its ion or appears to have when present in the combined state. It is also called oxidation state. Oxidation number of any atom in the elementary state is zero. Oxidation number of a monoatomic ion is equal to the charge on it. In compounds of metals with non-metals, metals have positive oxidation numbers while non-metals have negative oxidation numbers. In compounds of two difference elements, the more electronegative element has negative oxidation number whereas the other has positive oxidation number. In complex ions, the sum of the oxidation numbers of all the atoms is equal to the charge on the ion. If a compound contains two or more atoms of the same element, they may have same or different oxidation states according as their chemical bonding is same or different

The oxidation number (also called oxidation state) tells us the "charge" an atom appears to have in a compound or ion. Here's a simplified breakdown:

• Free elements: In their natural state, the oxidation number is always 0 (e.g., O₂, N₂, Fe).
• Monoatomic ions: The oxidation number is the same as the ion's charge. For example:
  • Na⁺ = +1
  • Cl^- = -1
• Metals and non-metals:
  • Metals usually have positive oxidation numbers (e.g., Na in NaCl is +1).
  • Non-metals typically have negative oxidation numbers (e.g., Cl in NaCl is -1).
• Electronegativity rule: In a compound with two different elements:
  • The more electronegative element gets a negative oxidation number.
  • The less electronegative element gets a positive oxidation number.
• Complex ions: The sum of oxidation numbers of all atoms equals the charge of the ion. For example:
  • In [SO₄]^2-, the total oxidation number is -2.
• Same element, different states: If a compound has more than one atom of the same element, those atoms can have the same or different oxidation states, depending on their bonds. For instance:
  • In H₂O₂, oxygen has an oxidation state of -1 (different from -2 in water, H₂O).

Pro Tip

To determine oxidation numbers, always:

• Start with known elements (e.g., H = +1, O = -2).
• Use the compound's charge or neutrality to calculate the unknowns.

This concept is key for balancing redox reactions and understanding how atoms interact in chemical bonding.