What is the oxidation state trend in transition elements?

Short Answer

Transition elements generally show multiple oxidation states, often differing by one or two units.

  • Early transition metals exhibit higher oxidation states due to more available d-electrons, while later elements prefer lower oxidation states.
  • The trend arises because both 4s and 3d electrons (or ns and (n-1)d for higher periods) participate in bonding, but stability of higher states decreases across the period.
Detailed Explanation :

Oxidation States in Transition Metals

Transition elements are defined as elements whose atoms or ions have partially filled d-orbitals.

  • General electronic configuration: (n-1)d¹⁻¹⁰ ns¹⁻²
  • Oxidation states arise from the loss of both ns and (n-1)d electrons, leading to multiple possible charges.

Early Transition Metals

  • Elements like Sc, Ti, V, Cr, Mn show high oxidation states.
  • Example:
    • Titanium (Ti) → +2, +3, +4
    • Manganese (Mn) → +2, +3, +4, +6, +7
  • Reason: Outer d- and s-electrons are easily removed → higher states stabilized by formation of covalent bonds and lattice energies.

Middle Transition Metals

  • Elements like Fe, Co, Ni exhibit moderate oxidation states, commonly +2 and +3.
  • Stability of higher states decreases due to increased nuclear charge and effective nuclear attraction, making full removal of electrons harder.

Late Transition Metals

  • Elements like Cu, Zn mostly show lower oxidation states:
    • Copper → +1, +2
    • Zinc → +2 (only one common state)
  • Reason: d-orbitals are nearly full → higher oxidation states are less stable.

Trend Across a Period

  1. Increasing Nuclear Charge:
    • Across a period, nuclear charge increases → higher oxidation states less stable.
  2. Decrease in Number of Unpaired d-Electrons:
    • Fewer unpaired electrons available for bonding → higher oxidation states become less common.
  3. Stability of Oxides and Halides:
    • Early elements form higher oxides (e.g., Mn₂O₇), while later elements form lower oxides (e.g., CuO).
  4. Formation of Complexes:
    • Multiple oxidation states allow transition metals to form varied coordination complexes, which are important in catalysis and biochemistry.

Trend Down a Group

  • Generally, similar oxidation states are retained, but higher states may be more stable for heavier elements due to increased orbital overlap.
  • Example: Iron group → Fe, Ru, Os → +2, +3 common; Os can also achieve +8 in OsO₄.

Significance of Oxidation State Trend

  1. Predicts Reactivity:
    • High oxidation states → strong oxidizing agents (e.g., Mn⁷⁺ in KMnO₄).
  2. Industrial Applications:
    • Used in catalysts, corrosion prevention, and metallurgy.
  3. Chemical Bonding:
    • Determines type of compounds formed, their color, magnetism, and solubility.
  4. Biological Importance:
    • Iron (+2, +3) and copper (+1, +2) oxidation states are crucial in enzymes and electron transport chains.
Conclusion

Transition elements show variable oxidation states, with higher states common in early elements and lower states dominating late elements. The trend is influenced by availability of d- and s-electrons, nuclear charge, and stability of compounds. Understanding this trend is crucial for predicting chemical reactivity, complex formation, and industrial applications of transition metals.