D and f- Block Elements
d- BLOCK ELEMENTS
The d-block elements are often called ‘Transition elements’ because their position in the periodic table is between the s- and p-block elements. The general electronic configuration is (n–1) d1–10ns0–2. In second and third transition series, there are more irregularities in the electronic configurations as compared to those in first transition series due to electron-electron and nuclear-electron interactions. Properties such as atomic radii, ionic radii, melting and boiling points, metallic properties, density, ionisation enthalpy, enthalpy of atomization vary along the period.
The filling of 4f before 5d orbital results in a regular decrease in the atomic radii which is known as lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number.
Transition metals of 3d series form stable halides. No regular trend in the E° (M2+/M) values is observed due to irregularity in ionisation enthalpies and sublimation enthalpies.
Transition metals differ widely in their chemical reactivity. Some of them are highly electropositive and dissolve in mineral acids. A few of them are noble, that is not affected by simple acids.
Transition elements are used as catalysts due to the presence of incomplete or empty d-orbitals, large surface area, variable oxidation state and ability to form complexes. Transition elements forms coloured compounds. Some of their compounds are diamagnetic while others are paramagnetic
The transition metals form a large number of complex compounds due to the comparatively smaller sizes of the metal ions, their high ionic charges and the availability of d orbitals for bond formation. Transition metals have voids or empty spaces in which atoms of Hydrogen, Carbon, Boron, Nitrogen etc. fit to form interstitial compounds.
Transition elements react with oxygen at elevated temperature to form oxides. Potassium dichromate and potassium permanganate are widely used for various purposes.
f- BLOCK ELEMENTS
In f-Block elements 4f and 5f-orbitals are progressively filled. Thus, these elements are known as f-block elements. The general electronic configuration is (n–2) f1–14 (n–1) d0–1 ns2. f-block consists of two series Lanthanoids and Actinoids
Elements in which the last electron enters in one of the 4f-orbitals are known as lanthanoids. These are silvery white in appearance and soft in touch. Lanthanoids have high melting points and are good conductors of heat and electricity.
Lanthanoid Contraction is a steady decrease in the atomic and ionic radii from lanthanum to lutetium. This occurs due to the Increase in the effective nuclear charge and poor screening effect of 4f-electrons.
Typical oxidation state of lanthanoids is +3. +2 and +4 are also exhibited by some of the elements. The first few elements of the series are quite reactive. With increasing atomic number, their behaviour becomes similar to that of aluminium.Lanthanoids are used for various purposes.
Elements in which the last electron enters one of the 5f-orbitals are known as actanoids. Metals with silvery appearance show great structural variability because of greater irregularities in their metallic radii.
The gradual decrease in the size of atoms or M3+ ions across the series is known as actanoid contraction. Actinoid Contraction is due to the increase in effective nuclear charge and poor screening effect of 5f-electrons. They show a large number of oxidation states. All the actinoid elements are radioactive. Actanoiods are used in various nuclear activities.