Class 12 Chemistry - CBSE
The d- and f-Block Elements
The d- and f-Block Elements chapter explores the transition elements in groups 3-12 and the inner transition elements (lanthanoids and actinoids) of the periodic table. For Class 12 CBSE board exams, this chapter is crucial as it carries significant weightage. You will study unique properties like variable oxidation states, coloured ions, catalytic behavior, and complex formation in d-block elements. Additionally, the chapter covers the lanthanoid contraction and its consequences in f-block elements, along with the preparation and properties of important compounds like potassium permanganate (KMnO4) and potassium dichromate (K2Cr2O7), which frequently appear in both theory and practical-based questions.
Start Learning FreeKey Concepts
Transition Elements
Elements that have incompletely filled d-orbitals in their ground state or in any of their oxidation states, generally classified as groups 3 to 12.
Lanthanoid Contraction
The steady decrease in atomic and ionic radii of lanthanoids with increasing atomic number, caused by the imperfect shielding of nuclear charge by 4f electrons.
Variable Oxidation States
Transition metals exhibit multiple oxidation states due to the very small energy gap between (n-1)d and ns orbitals, allowing electrons from both to participate in bonding.
Magnetic Moment
The magnetic behavior of transition metal ions is calculated using the spin-only formula based on the number of unpaired electrons.
Potassium Dichromate Preparation
Prepared from chromate ore (FeCr2O4) by fusion with sodium carbonate followed by acidification to form orange crystals of K2Cr2O7.
Important Formulas
Board Exam Info
This chapter typically carries around 3 to 5 marks in the CBSE Class 12 Chemistry board exam. Questions often include reasoning-type questions (such as why transition metals form colored compounds or act as good catalysts), calculations involving magnetic moments, and reactions/preparations of KMnO4 and K2Cr2O7.
Frequently Asked Questions
Why are transition metal compounds usually colored?
Transition metal compounds are colored due to d-d electron transitions, where an electron absorbs visible light to jump from one d-orbital to another, reflecting the complementary color.
What is lanthanoid contraction and what causes it?
It is the gradual decrease in atomic and ionic radii across the lanthanoid series. It is caused by the poor shielding effect of 4f electrons, which fails to offset the increasing nuclear charge.
Why do transition metals act as good catalysts?
They provide a large surface area with vacant orbitals and can exhibit variable oxidation states, allowing them to form intermediate complexes and lower the activation energy of reactions.
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