Oxidation state (oxidation number)
It is the apparent charge an atom shows in a compound, depending on how many electrons it loses, gains, or shares.
Trends in Oxidation States
1. Across a Period (left → right)
- Atoms on the left (metals) tend to lose electrons → show positive oxidation states.
- Example: Sodium usually shows +1, Magnesium +2, Aluminum +3.
- As we move right, elements start to gain or share electrons → show negative oxidation states.
- Example: Oxygen shows –2, Fluorine shows –1.
- The range of possible oxidation states increases across a period.
- Middle elements (like transition metals) can show multiple oxidation states.
2. Down a Group (top → bottom)
- The common oxidation state stays the same (because the number of valence electrons is the same).
- Example: All alkali metals show +1, all alkaline earth metals show +2.
- But for heavier elements, unusual oxidation states may appear due to the inert pair effect (the tendency of the outermost s-electrons not to participate in bonding).
- Example: Lead (Pb) shows +2 and +4, though +2 is more stable than +4 in heavier elements.
3. Special Case: Transition Metals
- Transition elements often show variable oxidation states because their d-electrons can also participate in bonding.
- Example: Iron shows +2 and +3, Manganese shows +2 to +7.
In summary:
- Across a period: Low positive → higher positive → negative values at the right.
- Down a group: Common oxidation state remains, but stability may change (inert pair effect for heavier elements).
- Transition metals: Show many oxidation states.