CBSE Class 12 Chemistry (2026–27)
Chapter 5: The d- and f-Block Elements
20 Important Questions and Answers
Q1. What are d-block elements? Mention their general electronic configuration.
Answer:
d-Block elements are those elements in which the last electron enters the d-orbital of the penultimate shell. They occupy Groups 3 to 12 of the periodic table and are commonly known as transition elements. Their general electronic configuration is ((n-1)d^{1-10}ns^{1-2}). Examples include iron (Fe), copper (Cu), and zinc (Zn). These elements exhibit characteristics such as variable oxidation states, coloured ions, catalytic activity, and the formation of complex compounds. However, not all d-block elements are transition elements. For example, zinc, cadmium, and mercury have completely filled d-orbitals in their common oxidation states and therefore do not show typical transition element properties.
Q2. Define transition elements. Why is zinc not considered a transition element?
Answer:
Transition elements are those elements that have partially filled d-orbitals in their atomic state or in one of their oxidation states. They are located in the d-block of the periodic table and exhibit properties such as variable oxidation states, coloured compounds, and catalytic activity. Zinc has the electronic configuration [Ar] 3d¹⁰4s². When it forms Zn²⁺ ions, its configuration becomes 3d¹⁰, which remains completely filled. Since neither zinc nor its common ion possesses a partially filled d-orbital, it does not satisfy the definition of a transition element. Hence, zinc is classified as a d-block element but not a transition element.
Q3. Why do transition elements show variable oxidation states?
Answer:
Transition elements exhibit variable oxidation states because the energies of the ns and (n−1)d orbitals are very close. As a result, electrons from both orbitals can participate in bond formation and chemical reactions. For example, iron exhibits +2 and +3 oxidation states, while manganese shows oxidation states ranging from +2 to +7. The availability of different numbers of electrons for bonding enables transition elements to form compounds with multiple oxidation states. This property contributes to their diverse chemistry and allows them to form a wide variety of compounds with different physical and chemical characteristics.
Q4. Why are transition metal compounds generally coloured?
Answer:
Transition metal compounds are generally coloured because of the presence of partially filled d-orbitals. When white light falls on these compounds, electrons absorb specific wavelengths of visible light and undergo d-d electronic transitions between energy levels split by surrounding ligands. The remaining transmitted or reflected light appears as the observed colour. For example, Cu²⁺ ions appear blue and Cr³⁺ ions appear green. Compounds of ions with completely filled or empty d-orbitals, such as Zn²⁺ and Sc³⁺, are usually colourless because d-d transitions are not possible. Thus, colour arises due to electronic transitions within partially filled d-orbitals.
Q5. Explain the catalytic properties of transition elements.
Answer:
Transition elements and their compounds act as effective catalysts because they can exhibit variable oxidation states and provide active surfaces for chemical reactions. Their ability to form intermediate compounds with reactants lowers the activation energy and increases the reaction rate. For example, finely divided iron is used in the Haber process for ammonia synthesis, vanadium pentoxide (V₂O₅) acts as a catalyst in the Contact process, and nickel is used in hydrogenation reactions. The presence of vacant d-orbitals also facilitates adsorption of reactants on the catalyst surface. These characteristics make transition metals important industrial catalysts.
Q6. What are interstitial compounds? Give examples.
Answer:
Interstitial compounds are formed when small atoms such as hydrogen, carbon, boron, or nitrogen occupy the vacant spaces or interstices within the crystal lattice of transition metals. These compounds retain the metallic properties of the parent metal but become harder and less reactive. They usually have high melting points and good electrical conductivity. Examples include titanium carbide (TiC), tungsten carbide (WC), and iron nitride (Fe₄N). These compounds are often non-stoichiometric because the small atoms do not occupy all available interstitial sites. Due to their hardness and strength, interstitial compounds are widely used in cutting tools and industrial applications.
Q7. What are alloy-forming properties of transition metals?
Answer:
Transition metals readily form alloys with one another because their atomic sizes are similar and they possess comparable crystal structures. An alloy is a homogeneous mixture of two or more metals that exhibits improved mechanical and chemical properties. Transition metal alloys are generally hard, strong, and resistant to corrosion. Examples include stainless steel (iron, chromium, and nickel), brass (copper and zinc), and nichrome (nickel and chromium). The alloy-forming tendency of transition metals is important in engineering and industrial applications because alloys often possess better strength, durability, and resistance to wear than pure metals.
Q8. What are lanthanoids? Mention their general electronic configuration.
Answer:
Lanthanoids are the fourteen elements that follow lanthanum in the periodic table, extending from cerium (Ce) to lutetium (Lu). In these elements, the differentiating electron enters the 4f subshell. Their general electronic configuration is [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Lanthanoids are known for their metallic nature, high reactivity, and predominantly +3 oxidation state. They are often called rare earth elements, although many are relatively abundant in nature. Lanthanoids have similar chemical properties due to the gradual filling of 4f orbitals, making their separation from one another difficult.
Q9. What is lanthanoid contraction?
Answer:
Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii of lanthanoid elements from cerium to lutetium with increasing atomic number. This occurs because the 4f electrons provide poor shielding of the increasing nuclear charge. As a result, the effective nuclear attraction on the outer electrons increases, pulling them closer to the nucleus. Lanthanoid contraction influences the properties of subsequent elements, leading to similarities in the sizes of elements such as zirconium and hafnium. It also affects the basicity of hydroxides and makes the separation of lanthanoids difficult due to their similar ionic sizes.
Q10. What are the consequences of lanthanoid contraction?
Answer:
Lanthanoid contraction has several important consequences. First, it causes the atomic and ionic radii of lanthanoids to decrease gradually across the series. Second, it results in the striking similarity between zirconium and hafnium, making them difficult to separate. Third, it decreases the basic strength of lanthanoid hydroxides from La(OH)₃ to Lu(OH)₃. Fourth, it contributes to the difficulty in separating lanthanoids because their ionic radii differ only slightly. Finally, it influences the physical and chemical properties of many elements following the lanthanoid series. Thus, lanthanoid contraction plays a significant role in inorganic chemistry.
Q11. Why do lanthanoids predominantly exhibit the +3 oxidation state?
Answer:
Lanthanoids predominantly show the +3 oxidation state because they can readily lose two 6s electrons and one 5d or 4f electron to attain a stable configuration. The +3 state is energetically the most favourable for almost all lanthanoids. Although some lanthanoids exhibit +2 or +4 oxidation states, these are less common and occur only when particularly stable electronic configurations are achieved. For example, Ce⁴⁺ and Eu²⁺ are relatively stable due to favourable electron arrangements. The predominance of the +3 oxidation state contributes to the chemical similarity observed among lanthanoid elements.
Q12. What are actinoids? Mention their general electronic configuration.
Answer:
Actinoids are the fourteen elements that follow actinium in the periodic table, ranging from thorium (Th) to lawrencium (Lr). In these elements, the differentiating electron enters the 5f subshell. Their general electronic configuration is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². Most actinoids are radioactive and many are synthetic elements. They exhibit a greater variety of oxidation states compared to lanthanoids due to the comparable energies of 5f, 6d, and 7s orbitals. Actinoids are important in nuclear science, with uranium and plutonium being widely used in nuclear reactors and energy production.
Q13. How do lanthanoids differ from actinoids?
Answer:
Lanthanoids and actinoids differ in several ways. Lanthanoids involve filling of the 4f orbitals, whereas actinoids involve filling of the 5f orbitals. Most lanthanoids are non-radioactive except promethium, while all actinoids are radioactive. Actinoids show a wider range of oxidation states because 5f electrons participate more readily in bonding. Lanthanoids predominantly exhibit the +3 oxidation state. Actinoids also form more complex compounds and display greater covalent character. Additionally, actinoid contraction is more pronounced than lanthanoid contraction. These differences arise from variations in electronic structure and orbital participation.
Q14. Why do actinoids show greater variability in oxidation states?
Answer:
Actinoids display greater variability in oxidation states because the energies of the 5f, 6d, and 7s orbitals are very similar. Consequently, electrons from all these orbitals can participate in chemical bonding. This enables actinoids to exhibit oxidation states ranging from +3 to +7 in some cases. For example, uranium commonly shows +3, +4, +5, and +6 oxidation states. In contrast, lanthanoids generally exhibit only the +3 oxidation state because their 4f electrons are less available for bonding. The variable oxidation states of actinoids contribute to their rich and complex chemistry.
Q15. Why are transition elements paramagnetic?
Answer:
Transition elements are generally paramagnetic because they possess one or more unpaired electrons in their d-orbitals. These unpaired electrons create magnetic moments that align with an external magnetic field, resulting in paramagnetism. The strength of paramagnetism depends on the number of unpaired electrons present. For example, Mn²⁺ has five unpaired electrons and exhibits strong paramagnetism. In contrast, ions with all electrons paired, such as Zn²⁺, are diamagnetic and are not attracted to a magnetic field. Thus, the magnetic behaviour of transition elements is directly related to their electronic configurations.
Q16. What are complex compounds? Why do transition metals form them readily?
Answer:
Complex compounds consist of a central metal atom or ion surrounded by molecules or ions called ligands. Transition metals readily form complexes because they possess small ionic sizes, high charge densities, and vacant d-orbitals that can accept electron pairs from ligands. Examples include [Fe(CN)₆]⁴⁻ and [Cu(NH₃)₄]²⁺. The formation of complexes enhances the stability of transition metal ions and contributes to their diverse chemistry. Complex compounds have important applications in analytical chemistry, medicine, metallurgy, and biological systems. The ability to form complexes is one of the characteristic properties of transition elements.
Q17. Explain the trend in melting points of transition elements.
Answer:
Transition elements generally have high melting points due to the strong metallic bonding resulting from the participation of both d and s electrons. As the number of unpaired d-electrons increases, metallic bonding becomes stronger, leading to higher melting points. The melting points increase from the beginning of a transition series, reach a maximum near the middle, and then decrease towards the end. Metals such as chromium and tungsten possess exceptionally high melting points because of their strong metallic bonds. Exceptions occur due to differences in crystal structures and electronic configurations. Overall, transition metals have much higher melting points than s-block elements.
Q18. Why are transition metals good conductors of heat and electricity?
Answer:
Transition metals are excellent conductors of heat and electricity because they contain a large number of delocalized electrons that move freely throughout the metallic lattice. These mobile electrons carry electrical charge and thermal energy efficiently. The overlap of d and s orbitals contributes to strong metallic bonding and facilitates electron mobility. As a result, metals such as copper, silver, and gold are widely used in electrical wiring and electronic devices. Their high conductivity, combined with mechanical strength and durability, makes transition metals extremely valuable in industrial and technological applications.
Q19. Why is KMnO₄ an important oxidizing agent?
Answer:
Potassium permanganate (KMnO₄) is a powerful oxidizing agent because manganese exists in its highest oxidation state of +7. It readily gains electrons and gets reduced to lower oxidation states, such as Mn²⁺ in acidic medium. During this reduction, it oxidizes other substances effectively. KMnO₄ is widely used in analytical chemistry for redox titrations and in water treatment and disinfecting processes. Its strong oxidizing nature arises from the high reduction potential of the permanganate ion. Due to its effectiveness and reliability, KMnO₄ is one of the most important oxidizing agents in chemistry.
Q20. Why is K₂Cr₂O₇ used as an oxidizing agent?
Answer:
Potassium dichromate (K₂Cr₂O₇) acts as a strong oxidizing agent, especially in acidic medium. In this process, chromium in the +6 oxidation state is reduced to Cr³⁺ while oxidizing other substances. The dichromate ion readily accepts electrons due to its high reduction potential. K₂Cr₂O₇ is commonly used in redox titrations, laboratory oxidations, and industrial chemical processes. Its solutions are stable and easy to prepare, making it a convenient oxidizing agent. The characteristic orange colour of dichromate changes to green during reduction, providing a visible indication of the reaction’s progress.