The d and f Block Elements Class 12 Notes Chemistry Chapter 8 - CBSE


What are d and f Block Elements ?


Transition Elements are the elements whose atoms or simple ions contain partially filled d-orbitals. These elements are called transition elements because they represent transition from the electropositive elements of s-block to electron negative elements of p-block. The f-block elements are those which involves the filling of f-orbitals of their third to the outermost shell. They are also called inner transition elements.

Electronic Configuration Of D-block Elements Or Transition Metals

The general configuration of d-block elements may be written as : [noble gas] (n – 1) d(1 – 10) ns1–2

Characteristics Of D-block Elements Or Transition Metals

  • Exhibit more than one oxidation state.
  • Many of their compounds are coloured.
  • They exhibit interesting magnetic properties.
  • They form an extensive series of compounds known as metal complexes or coordination compounds.

Magnetic Properties

  • Much of our understanding of transition metals comes from magnetic data.
  • Molecules with only closed shells of electrons have no inherent magnetic properties. But, when placed in a magnetic field, a small magnetic moment will be induced opposed to the field. These diamagnetic molecules are, therefore, repelled by the magnetic field.
  • Most transition metals do have unpaired electrons. These paramagnetic compounds do have magnetic properties resulting from both the spin and orbital motion of the unpaired electron. They are attracted into a magnetic field.
  • Origin of Paramagnetism from Classical Physics :

(a) Spin angular momentum : the spinning charge gives rise to a magnetic moment μ, in the direction perpendicular to the spin.

(b) Orbital angular momentum : due to the electron spinning around the nucleus.

  • This is a good analogy, but not a physical reality.
Magnetic Properties

Intersitial Compounds

Transition metals from compounds of indefinite structure and proportion which are called interstitial or non stoichiometric compounds. The reason is variable oxidation states are defects in their solid structures. Small atoms such as H, B, C and N can reside within the holes presents in the crystal lattice of transition elements.

Alloy Formation

The d-block elements have almost similar atomic sizes. Therefore, these elements can mutually substitute their positions in their crystal lattices. In this way, many alloys are possible between transition metals.

Metallic Character

All transition elements are metallic in nature.

Atomic and lonic Radii

  • Atomic and ionic radii decreases on moving from left to right
  • Atomic number increases : (i) Addition of new electrons in d subshell (ii) Effective nuclear charge increases shielding of d electron ineffective
  • Electrostatic attraction between nucleus and outermost electron increases
  • Towards end of series–slight increases in radii
  • Atomic radii increases down a group

Ionisation Energy

Due to filling in penultimate shell the increase in ionisation enthalpy along the period of d-block elements in very small.

Oxidation States

They have variable oxidation states due to the participation of ns and (n – 1) d-electrons.

Formation of complex compound

They are formed due to small size and high charge density of transition ions and vacant orbitals of a appropriate energy.

Catalytic Properties

Most of transition metals are used as catalyst because (i) presence of incomplete or empty d-orbitals, (ii) large surface area, (iii) variable oxidation state, (iv) ability to form complexes, e.g., Fe, Ni, V2O3, Pt, Mo, Co and used as catalyst.

Formation of Coloured Compounds

They form coloured ions due to the presence of incompletely filled d-orbitals and unpaired electrons, they can undergo d-d transition by absorbing colour from visible region and radiating complementary colour.

Lanthanoids And Its Electronic Configuration

The term ‘lanthanide’ was introduced by Victor Goldschmidt in 1925. Lanthanide series comprises the fifteen metallic chemical elements with atomic numbers 57 to 71, from lanthanum to lutetium. The valence shell electronic contiguration of the lanthanides is (n – 2)F1–14 (n – 1)d0–10 ns2

Oxidation State Of Lanthanoids

All the elemets in the lanthanide series show an oxidation state of + 3.

Lanthanides show variable oxidation states. They also show + 2, + 3 and + 4 oxidation states.

But the most stable oxidation states of Lanthanides is + 3. Elements in other states hence try to lose or gain electrons to get + 3 state.

By this the ions becomes strong reducing or oxidising agents respectively.

Uneven distribution of oxidation state among the metals is attributed to the high stability of empty, half-filled or fully filled f-sub shells.

Lanthanoid Contraction

Ln, Pm, Ho, Eb, Lu +3
Ce, Pr, Tb, Dy +3, +4
Sm, Eu, Tm, Yb +2, +3
Nd +2, +3, +4

Definition : The atomic and ionic size usually decrease from left to right across a period. This is due to increase in effective nuclear charge (Z*) which pulls the orbital electrons closer to the nucleus.

Cause of Lanthanide Contraction : In lanthanide atoms and ions, the 4f orbital is filled successively from Ce to Lu.

In general, the shielding effect of electrons decreases in the order ns > np > nf.

Expression : Z* = Z – S

Consequences Of Lanthanide Contraction

  • Due to the close similarity in electronic configuration the lanthanides have identical chemical properties.
  • The lanthanide contraction also explains the decreasing basicity of the lanthanides.
  • Certain pairs of elements such as Zr/Hf., Nb/Ta and Mo/W have almost identical size against the expected size increase due to increased atomic numbers.
  • This is a direct consequence of the lanthanide contraction.