Class 12 Chemistry - MAHARASHTRA
The d- and f-Block Elements
The chapter 'The d- and f-Block Elements' in Class 12 Chemistry explores the chemistry of transition and inner-transition elements as prescribed by the Maharashtra (MSBSHSE) board. Students learn about the electronic configurations, trends in atomic and ionic radii, ionization enthalpies, oxidation states, and colored ion formation of d-block elements (transition metals). The chapter also covers important compounds like potassium dichromate and potassium permanganate, alongside the lanthanoids and actinoids of the f-block, focusing on lanthanoid contraction. This is a high-yield chapter for board exams, frequently testing reasoning-based questions and chemical reactions.
Start Learning FreeKey Concepts
Transition Elements (d-block)
Elements that have incompletely filled d-orbitals in their ground state or in any of their oxidation states, generally classified under groups 3 to 12.
Variable Oxidation States
Due to very small energy differences between (n-1)d and ns orbitals, d-block elements exhibit multiple oxidation states by using electrons from both shells.
Color and Magnetic Properties
Transition metal ions are often colored due to d-d electron transitions and exhibit paramagnetism due to the presence of unpaired d-electrons.
Lanthanoid Contraction
The steady decrease in atomic and ionic radii of lanthanoids with an increase in atomic number, caused by the imperfect shielding of nuclear charge by 4f electrons.
Preparation of Potassium Permanganate (KMnO4)
Prepared commercially by the fusion of pyrolusite ore (MnO2) with KOH in the presence of air or an oxidizing agent like KNO3 to form potassium manganate, followed by electrolytic oxidation.
Important Formulas
Board Exam Info
In the Maharashtra (MSBSHSE) Class 12 Chemistry board exam, this chapter typically carries about 4 to 6 marks (with options). Questions commonly include reasoning-based problems (e.g., why transition metals form colored compounds or alloys), electronic configurations, preparation reactions of KMnO4 and K2Cr2O7, and consequences of lanthanoid contraction.
Frequently Asked Questions
Why do transition elements show variable oxidation states?
Transition elements show variable oxidation states because the energy difference between (n-1)d and ns orbitals is very small, allowing electrons from both energy levels to participate in bond formation.
What is lanthanoid contraction and what are its consequences?
It is the gradual decrease in ionic radii across the lanthanoid series due to poor shielding by 4f electrons. Consequences include the identical radii of 4d and 5d series elements (e.g., Zr and Hf), making them difficult to separate.
Why are compounds of transition metals generally colored?
Transition metal compounds are colored due to d-d electronic transitions, where an electron absorbs visible light to jump from one d-orbital to another, transmitting the complementary color.
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