The d- and f-Block Elements — Class 12 Chemistry

The d-block consists of groups 3 to 12, in which the d orbitals are progressively filled across four long periods; these are the transition metals. The f-block, the inner transition metals, comprises the lanthanoids (filling 4f) and actinoids (filling 5f), placed separately at the foot of the periodic table. A transition metal is one with an incomplete d subshell in the atom or its common ions, so zinc, cadmium and mercury are excluded. Transition elements show characteristic properties: high melting points and densities from strong metallic bonding involving d electrons, variable oxidation states because and electrons have similar energies, formation of coloured ions due to d-d electronic transitions, strong tendency to form complex ions, paramagnetism from unpaired d electrons, and catalytic activity. Their atomic radii are nearly constant across a series. The lanthanoids are remarkably similar to one another, and the steady decrease in size across the series, the lanthanoid contraction, makes the second and third transition series alike in size. Actinoids show a wider range of oxidation states and are all radioactive. Important compounds include potassium dichromate and potassium permanganate, strong oxidising agents prepared from their ores.