The elements in which last electron enters the f-orbital of the atoms are called f-block elements. These elements are also called inner transition elements. They consist of two series of elements placed at the bottom of the periodic table. These two series are generated by the filling of characteristic electrons in the 4f- and 5f- orbitals.
Chemistry of Lanthanoids
The series involving the filling of 4f-orbitals following lanthanum La (Z=57) is called Lanthanoid series. The elements presents in the series are called lanthanoids. There are fourteen elements in this series starting with cerium Ce (Z=58) and ending with lutetium Lu (Z=71). These elements occur very rarely and therefore these elements have also been called rare earth elements.
General characterstics of Lanthanoids:-
- All lanthanoids exhibit a common stable oxidation state of +3. In addition some lanthanoids show +2 and +4 oxidation states also.
- The first ionization enthalpies of the lanthanoids are of the order of 600 kj mol-1 and second ionization enthalpies are about 1200 kj mol-1 , comparable with those of calcium.
- Some of the trivalent ions of lanthanoids are coloured in the solid state as well as in the solution.
- Ions which contain all paired electrons are diamagnetic while those containing unpaired electrons are paramagnetic. Among the lanthanoids, La3+ and Lu3+, which have 4f0 or 4f14 configurations are diamagnetic and all other trivalent lanthanoids ions are paramagnetic because they have unpaired electrons.
- All the lanthanoids are highly electropositive metals and have almost similar chemical reactivity. Because of their similar chemical reactivities, their separation from one another is very difficult. The lanthanoids are, in general, more reactive than d-block elements.
- All the lanthanoids are silvery white soft metals which tarnish readily on exposure to air.
- The hardness of lanthanoids increases with increasing atomic number. Samarium is very hard like steel.
- They have typical metallic structure and are good conductors of heat and electricity.
- In the lanthanoids series, with increasing atomic number, the atomic and ionic radii decrease from one element to another. The steady decrease in atomic and ionic sizes of lanthanide elements with increasing atomic number is called lanthanoid contraction.
Uses of Lanthanoids:-
These metals are extracted as mixture of alloys. Lanthanoids are specially used for the production of alloy steels for plates and pipes. These are also called mischmetals.
Chemistry of Actinoids
The series involving the filling of 5f-orbitals is called actinoids series. It follows actinium, Ac (Z-89) and the elements presents in the series are called actinoids. They include the elements form thorium, Th (Z-90) to lawrencium, Lw (Z-103). As in case of lanthanoids, the name of the series is after the preceding element actinium with which these elements closely resemble. This series also consist of 14 elements.
General characterstics of Actinoids:-
- The melting and boiling points of actinoids are moderately high but they are considerably lower than those of transition elements.
- The densities of actinoids first increase than decrease.
- Actinoids show different oxidation state such as +2, +3, +4, +5, +6 and +7. However +3 oxidation state is most common among all the actinoids.
- Like lanthanoids, actinoids also show actinoids contraction. The size of the atoms or ions of actinoids decrease regularly along the series with the increase in atomic number from actinium to lawrencium. This steady decrease in the ionic radii with the increase in atomic number is called actinoid contraction.
- Most of the actinoid ions are paramagnetic because they contain unpaired electrons.
- Actinoids ions are generally coloured.
- The actinoid metals are all silvery in appearance & display a variety of structures.
- These are highly reactive metals especially in finally divided form.