Oxidation State of Transition Metals - NEET Notes, Formula & Common Mistakes

What NEET Asks

• NEET frequently tests the calculation of oxidation states in various compounds and complex ions involving transition metals.
• Questions often involve identifying reasons for variable oxidation states or comparing the stability of different oxidation states.
• Expect 1-2 direct questions from Redox Reactions, often including d-block elements.

Key Points

• Transition metals exhibit variable oxidation states due to the involvement of both (n-1)d and ns electrons in bonding.
• The most common oxidation state for first-row transition metals is +2 (from the loss of 4s electrons).
• Higher oxidation states are achieved by involving the (n-1)d electrons.
• The highest oxidation state generally corresponds to the sum of ns and (n-1)d electrons (up to Group 7, i.e., Mn).
• Stability of oxidation states varies across the period and down the group (e.g., higher OS more stable for heavier elements).
• Oxidation state of an element in its uncombined state is always zero.
• Standard oxidation states: Group 1 (+1), Group 2 (+2), F (-1), O (-2, except in peroxides -1, superoxides -1/2, OF2 +2), H (+1, except in metal hydrides -1).

Must-Know Formula / Reaction

Sum of oxidation states = Total charge on the species

• ∑(Oxidation state of each atom × number of atoms) = Charge
  • Example: For Cr in K₂Cr₂O₇: 2(+1) + 2(Cr) + 7(-2) = 0 2 + 2Cr - 14 = 0 2Cr = 12 Cr = +6

Common Mistakes

• Students often forget the charge of common ligands (e.g., CN⁻ is -1, H₂O is 0) when calculating oxidation states in coordination compounds.
• Don't confuse variable oxidation states of transition metals with fixed oxidation states of elements like Group 1 or 2 metals.
• Assuming oxygen is always -2; overlooking exceptions like peroxides (H₂O₂, Na₂O₂) or superoxides (KO₂).

Rapid Revision

Transition metals show variable OS due to (n-1)d and ns electron participation. Calculate OS by setting the sum of individual OS to the total charge. Remember exceptions for O and H. Highest OS often matches total d+s electrons up to Mn.