What NEET Asks
- Direct calculation: Questions frequently ask for the oxidation state of a specific element in a given compound or ion.
- Conceptual understanding: Identifying correct/incorrect statements related to oxidation state rules and exceptions.
- Application to redox reactions: Using oxidation states to determine if a reaction is redox and to identify oxidizing/reducing agents.
- Typically 1-2 questions from Redox Reactions chapter, carrying 4-8 marks.
Key Points
- Sum of O.S.: In a neutral molecule, the algebraic sum of oxidation states of all atoms is zero. For a polyatomic ion, it equals the charge on the ion.
- Free elements: Oxidation state of an element in its free, uncombined state (e.g., O2, S8, Fe) is zero.
- Group 1 & 2 metals: Alkali metals (Group 1) are always +1, alkaline earth metals (Group 2) are always +2 in compounds.
- Fluorine: Always -1 in its compounds due to its high electronegativity.
- Hydrogen: +1 with non-metals (e.g., HCl, H2O), -1 with metals (metal hydrides, e.g., NaH, CaH2).
- Oxygen: Usually -2. Exceptions: -1 in peroxides (H2O2, Na2O2), -1/2 in superoxides (KO2), +2 in OF2, +1 in O2F2.
- Halogens (Cl, Br, I): Usually -1, but can be positive when combined with more electronegative elements like oxygen or fluorine.
- Maximum O.S.: For s & p-block elements, it's generally equal to their group number (e.g., S can be +6 in H2SO4).
Must-Know Formula / Reaction
Σ (O.S. of each atom) = Total charge of molecule/ion
- Σ: Summation
- O.S.: Oxidation state of each individual atom
- Total charge: Zero for a neutral molecule, or the charge on a polyatomic ion.
Common Mistakes
- Students often forget the specific exceptions for oxygen's oxidation state (peroxides, superoxides, OF2).
- Don't confuse the overall charge of an ion with the oxidation state of a single element within it.
- Blindly applying standard O.S. rules without considering structural aspects (like peroxo linkages) when calculated O.S. exceeds maximum possible.
Rapid Revision
Master standard O.S. rules for common elements (H, O, F, Group 1/2). Always equate the sum of O.S. to the overall charge. If an unusually high O.S. is calculated, immediately suspect peroxide or other unusual bonding and consider the structure.