What is the primary reason why transition metals exhibit variable oxidation states in their compounds?

💡 Solution & Explanation

Transition metals exhibit multiple oxidation states because the energy difference between their outermost ns and penultimate (n-1)d orbitals is exceptionally small, allowing electrons from both levels to participate in chemical bonding. Elements with completely filled or empty d-orbitals generally lack this versatility. This unique electronic configuration also accounts for their characteristic catalytic activity and colored ion formation.