The d and f Block Elements — Study Notes

1. Introduction

The d-block elements, commonly called transition elements, occupy the middle of the periodic table between the s-block and the p-block. In these elements, the last electron enters the d-orbital of the penultimate shell. They include elements of groups 3 to 12 and exhibit properties that are uniquely characteristic of transition metals, such as variable oxidation states, coloured compounds, catalytic activity, and the formation of complexes. The chapter also covers the f-block elements, which include the lanthanoids and actinoids, whose last electron enters the f-orbital.

The term transition element refers to the elements in groups 3 to 12 that have partially filled d-orbitals either in the ground state or in one of their common oxidation states. The general electronic configuration is (n-1)d^1-10 ns^1-2. The d-block elements are all metals, mostly hard and dense, with high melting and boiling points. Their properties arise directly from the presence of the partially filled d-orbitals, which allow the formation of a wide variety of oxidation states and coloured species.

The chapter begins with the electronic configurations and the observed trends in atomic and ionic radii, ionisation enthalpies, and oxidation states across the first transition series. It then explains the magnetic properties, the origin of colour in transition metal compounds, and the catalytic properties. The final sections describe the lanthanoids and actinoids, their electronic configurations, the lanthanoid contraction, and their industrial and nuclear applications.

2. Electronic Configurations and Properties

The general electronic configuration of d-block elements is (n-1)d^1-10 ns^1-2. There are three transition series: the first series (Sc to Zn, 3d), the second series (Y to Cd, 4d), and the third series (La, Hf to Hg, 5d). The fourth series involves the 6d orbitals and is incomplete.

Some exceptions exist: chromium is [Ar] 3d^5 4s^1 and copper is [Ar] 3d^10 4s^1, because half-filled and fully filled d-orbitals confer extra stability due to exchange energy and symmetry.

Atomic and Ionic Radii

In the first transition series, the atomic radius decreases from Sc to Cr but remains nearly constant from Cr to Cu, before increasing for Zn. This happens because the additional electrons enter inner d-orbitals, which do not greatly affect the size. The ionic radii follow a similar pattern, decreasing with the increase in oxidation state.

Ionisation Enthalpies

The ionisation enthalpies of transition metals are higher than those of s-block elements but lower than those of p-block elements. They increase gradually across the series. The relatively low ionisation enthalpies of the first transition series allow a variety of oxidation states.

Oxidation States

Transition metals show a variety of oxidation states because of the small energy difference between the (n-1)d and ns orbitals, allowing electrons from both to participate in bonding. For example, manganese shows oxidation states from +2 to +7, and the +2 and +3 states are the most common for the first transition series. The highest oxidation state of the elements in the first series increases up to manganese (+7) and then decreases.

Metallic Character

Transition metals are hard, dense, and have high melting points because of strong metallic bonding involving the d-electrons. They are good conductors of heat and electricity.

3. Trends in Chemical Properties

Formation of Complexes

Transition metals readily form coordination complexes because of their small size, high charge, and vacant d-orbitals of suitable energy to accept electron pairs from ligands.

Colour

Transition metal compounds are generally coloured. The colour arises from the d-d transition of electrons between the split d-orbitals in the crystal field of the ligands. For example, [Ti(H2O)6]3+ is violet, [Cu(H2O)6]2+ is blue, and KMnO4 is violet. The intensity of the colour depends on the number of unpaired electrons.

Magnetic Properties

Substances with unpaired electrons are paramagnetic, and the magnetic moment is given by: $$\mu = \sqrt{n(n+2)} \ \text{BM}$$ where $n$ is the number of unpaired electrons. The spin-only magnetic moment is expressed in Bohr magnetons. For example, Cu2+ (one unpaired electron) has a magnetic moment of about 1.73 BM.

Catalytic Properties

Transition metals and their compounds are excellent catalysts. For example, iron is used in the Haber process, nickel in hydrogenation, and V2O5 in the Contact process. Their catalytic activity arises from their ability to adopt variable oxidation states and to adsorb reactants on their surface.

Interstitial Compounds

Transition metals form interstitial compounds in which small atoms like H, C, and N occupy the interstitial spaces in the metal lattice. These compounds are hard, have high melting points, and retain metallic character.

Alloy Formation

Transition metals form alloys readily because their atomic radii are similar, allowing atoms to substitute for one another in the crystal lattice.

4. The Lanthanoids

The lanthanoids are the fourteen elements from cerium to lutetium in which the 4f orbitals are progressively filled. Their general electronic configuration is [Xe] 4f^1-14 5d^0-1 6s^2.

Lanthanoid Contraction

The steady decrease in the atomic and ionic radii of the lanthanoids from La3+ to Lu3+ is called lanthanoid contraction. It is caused by the imperfect shielding of the 4f electrons. The consequences of this contraction are significant: 1. The radii of the second and third transition series are similar, causing zirconium and hafnium to have nearly identical properties and making their separation difficult. 2. The basic character of lanthanoid hydroxides decreases from La(OH)3 to Lu(OH)3.

Oxidation States

The common oxidation state of lanthanoids is +3. Some show +2 and +4 states; for example, Eu2+ and Ce4+ are stable because of the stability of half-filled and fully filled f-orbitals.

Uses

Lanthanoids are used in mixed metal alloys for lighter flints, in catalysts, in phosphors for television screens, and in the manufacture of strong permanent magnets such as samarium-cobalt magnets.

5. The Actinoids

The actinoids are the elements from thorium to lawrencium in which the 5f orbitals are progressively filled. Their general electronic configuration is [Rn] 5f^1-14 6d^0-1 7s^2.

Actinoids show a wider range of oxidation states than lanthanoids because the 5f orbitals have higher energy and can participate more easily in bonding. The +3 and +4 states are common, but states up to +7 are observed for some elements such as neptunium and plutonium. Actinoids are radioactive, and the later members have short half-lives that make them difficult to study.

Actinoid contraction, similar to lanthanoid contraction, is observed because of the poor shielding of the 5f electrons. The actinoids are used in nuclear energy generation; for example, uranium-235 undergoes fission to produce energy, and plutonium is used in nuclear reactors and weapons.

Quick Revision Tables

Table 1: Properties of the First Transition Series

Table 2: d-Block versus f-Block Elements

Mind Map

Important Diagrams (SVG)

Diagram 1: d-Orbital Splitting and Colour

Diagram 2: Lanthanoid Contraction

Common Mistakes

1. Forgetting that zinc (3d10 4s2) is not strictly a transition metal because its d-orbitals are completely filled in both the ground state and the common +2 state.
2. Believing that all transition metal compounds are coloured; Sc3+ and Zn2+ with no unpaired d-electrons give colourless aqueous ions.
3. Miswriting the exceptions to configuration: Cr is 3d5 4s1 and Cu is 3d10 4s1, not 3d4 4s2 and 3d9 4s2.
4. Confusing lanthanoid contraction with actinoid contraction; both occur but the lanthanoid series is better studied because the actinoids are radioactive and short-lived.
5. Using the wrong magnetic moment formula; the spin-only moment is mu = sqrt(n(n+2)) BM where n is the number of unpaired electrons.
6. Believing that the +7 oxidation state is common for all transition metals; only elements like Mn, Cr, and V show such high states.
7. Saying lanthanoids have a single oxidation state; Eu2+ and Ce4+ show +2 and +4 states because of stable f-configurations.

Exam Tips

1. Memorise the electron configurations of the first transition series and the exceptions (Cr and Cu).
2. Remember that the colour and magnetic moment depend on the number of unpaired electrons; Zn2+ and Sc3+ are colourless.
3. Learn one example for each property: complex formation (Fe in haemoglobin), catalysis (Fe in Haber, V2O5 in Contact), interstitial compounds (steel, cementite).
4. For lanthanoid contraction, always link it to the poor shielding of 4f electrons and the similar radii of Zr and Hf.
5. Know that KMnO4 (Mn in +7 state) and K2Cr2O7 (Cr in +6 state) are strong oxidising agents with distinct colours.
6. Practise the relation between oxidation state and the number of electrons in the d-orbital; highest oxidation state usually equals the group number minus two for most d-block elements.

Conclusion

The d and f block elements demonstrate how the filling of inner orbitals shapes macroscopic properties. The partially filled d-orbitals of transition metals give rise to variable oxidation states, coloured ions, magnetic behaviour, and powerful catalysis, making them indispensable in industrial chemistry, biology, and materials science. The lanthanoids and actinoids, with their f-orbital electrons, exhibit unique contraction effects and radioactive behaviour that underpin the nuclear energy industry. The nearly identical properties of second and third series metals such as Zr and Hf, caused by lanthanoid contraction, illustrate the subtle interplay between electronic configuration and atomic size. Mastery of this chapter provides a deeper appreciation of the periodic table and prepares students for the study of coordination compounds, where the d-block metals play a central role.

Test Your Understanding

1. Why are transition metals able to show variable oxidation states?
2. Why is zinc not regarded as a transition element?
3. Give a reason for the colour of transition metal compounds and name one colourless aqueous ion.
4. What is lanthanoid contraction and what are its main consequences?
5. Explain why transition metals and their compounds act as good catalysts.