What NEET Asks
- Oxidation state calculations for various elements in compounds/ions are frequently tested.
- Identifying oxidizing/reducing agents often relies on correctly determining oxidation states.
- Questions can involve disproportionation reactions or complex redox reactions where oxidation states are crucial.
Key Points
- Elemental State: The oxidation state of an element in its uncombined or elemental form is always zero (e.g., O₂, Na, Fe).
- Monoatomic Ions: Oxidation state equals the charge on the ion (e.g., Na⁺ = +1, Cl⁻ = -1).
- Sum in Neutral Compounds: The algebraic sum of oxidation states of all atoms in a neutral compound is zero.
- Sum in Polyatomic Ions: The sum of oxidation states of all atoms in a polyatomic ion equals the net charge of the ion.
- Group 1 & 2 Metals: Alkali metals (Group 1) are always +1, and Alkaline earth metals (Group 2) are always +2.
- Fluorine: Always -1 in all its compounds due to its highest electronegativity.
- Hydrogen: Usually +1. Exception: -1 in metal hydrides (e.g., NaH).
- Oxygen: Usually -2. Exceptions: -1 in peroxides (e.g., H₂O₂), -1/2 in superoxides (e.g., KO₂), +2 in oxygen difluoride (OF₂).
Must-Know Formula / Reaction
To find an unknown oxidation state (X) for an element in a compound/ion: Sum of (number of atoms × oxidation state of each atom) = Total charge of compound/ion.
- Example: For H₂SO₄, (2 × OS of H) + (1 × OS of S) + (4 × OS of O) = 0.
Common Mistakes
- Students often forget exceptions for Hydrogen and Oxygen (e.g., peroxides, metal hydrides, OF₂), leading to incorrect calculations.
- Don't confuse the oxidation state of a polyatomic ion with the oxidation state of a specific element within it.
- Assuming constant oxidation states for transition metals; always calculate them based on the compound's overall charge.
Rapid Revision
Remember: Element = 0. Group 1 = +1, Group 2 = +2, F = -1. H is +1 (except metal hydrides). O is -2 (except peroxides, superoxides, OF₂). Sum of OS = total charge. Master these for quick, accurate calculations.