The d and f Block Elements Class 12 Notes Chemistry Chapter 8 - CBSE
Chapter:8
What are d and f Block Elements ?
Introduction
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
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.
General Trends In Properties Of The Transition Metals
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.
Lanthanoids And Its Electronic Configuration
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.
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