Introduction to NCERT Exemplar Solutions for Class 12 Chemistry Chapter 8: The D-Block and F-Block Elements

The d-block and f-block elements are often referred to as transition and inner transition elements. These elements occupy the central portion of the periodic table and are characterized by the progressive filling of d and f orbitals. They exhibit unique properties such as variable oxidation states, formation of colored ions, complex formation, and catalytic behavior. Chapter 8 introduces students to these special groups of elements, emphasizing their properties, trends, and important compounds. The Class 12 NCERT Exemplar Solutions for this chapter offer detailed problem-solving methods that help learners understand the complex behavior of these elements.

D-Block Elements: Transition Metals

The d-block consists of groups three to twelve. These metals are called transition elements because they show a gradual transition in properties from highly reactive metals on the left to less reactive non-metals on the right side of the periodic table.

Key characteristics include:

Variable oxidation states: Due to the close energy of (n-1)d and ns orbitals, transition metals exhibit multiple oxidation states. For example, iron shows +2 and +3 states.
Formation of colored ions: The presence of partially filled d orbitals leads to electronic transitions that give rise to color. For instance, copper(II) sulfate solution is blue.
Catalytic properties: Many transition metals and their compounds act as catalysts in industrial processes. For example, iron in the Haber process and vanadium pentoxide in the contact process.
Magnetic properties: Depending on unpaired electrons, they may exhibit paramagnetism or ferromagnetism.

Exemplar questions test students’ ability to explain these properties with reasoning, calculate oxidation states, and predict trends across the series.

Important Compounds of Transition Metals

The chapter highlights compounds like potassium dichromate (K₂Cr₂O₇) and potassium permanganate (KMnO₄). Both are strong oxidizing agents with industrial and laboratory applications. Exemplar questions often involve balancing redox reactions using these compounds in acidic, basic, or neutral media.

F-Block Elements: Inner Transition Metals

The f-block elements are divided into lanthanoids (atomic numbers 57 to 71) and actinoids (atomic numbers 89 to 103). These elements are characterized by the filling of 4f and 5f orbitals.

Lanthanoids: Known for their similar chemical properties and gradual decrease in atomic size across the series, called the lanthanoid contraction. This contraction influences the Chemistry of post-lanthanide elements and transition metals.
Actinoids: Most are radioactive, and some such as uranium and thorium are important in nuclear energy production. They exhibit a wide range of oxidation states due to the close energies of 5f, 6d, and 7s orbitals.

Exemplar questions encourage students to compare lanthanoids and actinoids, explain the reasons for contraction, and discuss the uses of elements like uranium in nuclear reactors.

General Trends in D- and F-Block Elements

Students learn about periodic variations such as atomic and ionic radii, ionization enthalpy, and standard electrode potential across these blocks. The irregularity in trends is due to the special stability of half-filled and fully filled d and f subshells. Exemplar problems often ask students to rationalize such irregularities with proper reasoning.

Applications of Transition and Inner Transition Elements

These elements play significant roles in daily life and industry: