NCERT Class 12 Chemistry Solutions The d and f Block Elements

NCERT Class 12 Chemistry Solutions The d and f Block Elements - NCERT Solutions prepared for CBSE students by the best teachers in Delhi. 

Class XII  Chemistry

Chapter 8 – The d and f Block Elements

Question 1:

What is lanthanoid contraction? What are the consequences of lanthanoid contraction?


As we move along the lanthanoid series, the atomic number increases gradually by one. This means that the number of electrons and protons present in an atom also increases by one. As electrons are being added to the same shell, the effective nuclear charge increases. This happens because the increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron. Also, with the increase in atomic number, the number of electrons in the 4f orbital also increases. The 4f electrons have poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases. This results in a steady decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.

Consequences of lanthanoid contraction

(i) There is similarity in the properties of second and third transition series.

(ii) Separation of lanthanoids is possible due to lanthanide contraction.

(iii) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)3 to Lu(OH)3.)


Question 2:

What are the characteristics of the transition elements and why are they called transition elements? Which of the d-block elements may not be regarded as the transition elements?


Transition elements are those elements in which the atoms or ions (in stable oxidation state) contain partially filled d-orbital. These elements lie in the d-block and show a transition of properties between s-block and p-block. Therefore, these are called transition elements.

Elements such as Zn, Cd, and Hg cannot be classified as transition elements because these have completely filled d-subshell.


Question 3:

In what way is the electronic configuration of the transition elements different from that of the non-transition elements?


Transition metals have a partially filled d−orbital. Therefore, the electronic configuration of transition elements is (n − 1)d1-10 ns0-2.

The non-transition elements either do not have a d−orbital or have a fully filled d−orbital. Therefore, the electronic configuration of non-transition elements is ns1-2 or ns2 np1-6.


Question 4:

What are the different oxidation states exhibited by the lanthanoids?


In the lanthanide series, +3 oxidation state is most common i.e., Ln(III) compounds are predominant. However, +2 and +4 oxidation states can also be found in the solution or in solid compounds.


Question 5:

Explain giving reasons:

(i) Transition metals and many of their compounds show paramagnetic behaviour.

(ii) The enthalpies of atomisation of the transition metals are high.

(iii) The transition metals generally form coloured compounds.

(iv) Transition metals and their many compounds act as good catalyst.


(i) Transition metals show paramagnetic behaviour. Paramagnetism arises due to the presence of unpaired electrons with each electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. However, in the first transition series, the orbital angular momentum is quenched. Therefore, the resulting paramagnetism is only because of the unpaired electron.

(ii) Transition elements have high effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomization of transition metals is high.

(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from visible light region to promote an electron from one of the  d−orbitals to another. In the presence of ligands, the d-orbitals split up into two sets oforbitals having different energies. Therefore, the transition of electrons can take place from one set toanother. The energy required for these transitions is quite small and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength and the rest is reflected, imparting colour to the solution.

(iv) The catalytic activity of the transition elements can be explained by two basic facts.

(a) Owing to their ability to show variable oxidation states and form complexes, transition metals form unstable intermediate compounds. Thus, they provide a new path with lower activation energy, Ea, for the reaction.

(b) Transition metals also provide a suitable surface for the reactions to occur.


Question 6:

What are interstitial compounds? Why are such compounds well known for transition metals?


Transition metals are large in size and contain lots of interstitial sites. Transition elements can trap atoms of other elements (that have small atomic size), such as H, C, N, in the interstitial sites of their crystal lattices. The resulting compounds are called interstitial compounds.


Question 7:

How is the variability in oxidation states of transition metals different from that of the non-transition metals? Illustrate with examples.


In transition elements, the oxidation state can vary from +1 to the highest oxidation state by removing all its valence electrons. Also, in transition elements, the oxidation states differ by 1 (Fe2+ and Fe3+; Cu+ and Cu2+). In non-transition elements, the oxidation states differ by 2, for example, +2 and +4 or +3 and +5, etc.


Students should free download the NCERT solutions and get better marks in exams. panel of teachers recommend students to practice questions in NCERT books and download NCERT solutions.



Click to View or Download pdf file
Click for more Chemistry Study Material

Latest NCERT & CBSE News

Read the latest news and announcements from NCERT and CBSE below. Important updates relating to your studies which will help you to keep yourself updated with latest happenings in school level education. Keep yourself updated with all latest news and also read articles from teachers which will help you to improve your studies, increase motivation level and promote faster learning

BRICS International Online Mathematics Competition

BRICSMATH.COM is an annual International Online Competition in Mathematics, for students of classes I – XII of 07 BRICS countries (Brazil, Russia, India, China and South Africa, Indonesia and Vietnam). The competition is held online on the website and...

How to Effectively Answer CBSE Board Examination Question Papers

The intend of this article is to share the best ways to answer the CBSE Board Examination. Usually most of the students tend to become nervous at the times of the board examination. In this article we will share the most effective ways to make the answers to the CBSE...

Tricks for Utilization of additional time introduced in CBSE Board Exams

Tricks for Proper Utilization of Add-On 15 Minutes introduced in CBSE Board Exams The intend of this article is to share the best suggestions and guidelines to utilize the extra 15 minutes provided for reading the question paper in CBSE Board Examination. These extra...

Celebration of Republic Day by CBSE

As you are aware, Republic Day is celebrated every year on 26th January to honour the Constitution and Republic of our country. Celebration of this occasion annually helps youth and children of our country to be aware of the significance of Indian Constitution, unity...

Digital Marks Sheets Migration Certificates and Pass Certificates

CBSE pioneered in providing digital academic documents through its academic repository called “Parinam Manjusha” and DigiLocker.  As on now result data from 2004 till this year is available in the repository.  Approx. 12 crores digital academic documents are available...

The CBSE Science Challenge 2020

The CBSE Science Challenge - 2020 Science is inexplicably linked with our lives and helps us to understand the world around us better. Scientific and technological developments contribute to progress and help improve our standards of living. By engaging with this...

Studies Today