The elements in which the last electron enters the f-orbital of the atoms are called f-block elements. In these elements, the last electron is added to the third to the outermost (called antepenultimate) energy level i.e. (n-2)f.
Their general electronic configuration is
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.
General Characteristics of Lanthanoids
1) Electronic configuration
The lanthanoids have electronic configuration with 6s2 filled and involve the gradual filling of 4f-orbitals. The energies of d-orbitals (5d) and the next inner shell f-orbitals (4f) are closely similar and, therefore, the order of filling the 4f-orbitals in the atoms shows quite irregularities.
|Lanthanum||La||57||[Xe] 5d1 6s2|
|Cerium||Ce||58||[Xe] 4f1 5d16s2|
|Praseodymium||Pr||59||[Xe] 4f3 5d06s2|
|Promethium||Pm||61||[Xe] 4f5 5d06s2|
|Samarium||Sm||62||[Xe] 4f6 5d06s2|
|Europium||Eu||63||[Xe] 4f7 5d16s2|
|Gadolinium||Gd||64||[Xe] 4f7 5d16s2|
|Terbium||Tb||65||[Xe] 4f9 5d06s2|
|Dysprosium||Dy||66||[Xe] 4f7 5d16s2|
|Holmium||Ho||67||[Xe] 4f11 5d06s2|
|Erbium||Er||68||[Xe] 4f12 5d06s2|
|Thulium||Tm||68||[Xe] 4f13 5d06s2|
|Ytterbium||Yb||70||[Xe] 4f14 5d06s2|
|Lutetium||Lu||71||[Xe] 4f14 5d16s2|
2) Oxidation states
All lanthanoids exhibit a common stable oxidation state of +3. In addition some lanthanoids show +2 and +4 oxidation states also in solution or in solid Compounds. These are shown by those elements which by doing so attain the stable f0 (empty f-subshell), f7 (half filled f-subshell) and f14 (filled f-subshell) configurations.
(i) Ce and Tb exhibit +4 oxidation states: Cerium (Ce) and terbium (Tb) attain f0 and f7 configurations respectively when they get +4 Oxidation state,
(iii) La, Gd and Lu exhibit only +3 oxidation states. These elements show +3 oxidation states only because by losing three electrons, they acquire stable configurations of empty, half-filled and completely filled 4f-subshells.The stability of different oxidation states has strong effect on the properties of these elements.
For example, Ce (IV) is favoured because of its noble gas configuration. But it is a strong oxidant changing to common +8 oxidation state.
Pr, Nd, Tb and Dy also show +4 oxidation states but only in oxides MO2.
Similarly, Eu2+ is stable because of its half filled 4f7 configuration. However, it is a strong reducing agent changing to Eu3+ (common oxidation state).
Similarly, Yb2+ having the configuration 4f14 is a reductant. Tb IV has half filled f-orbitals and is an oxidant. Samarium also behaves like europium exhibiting both +2 and +3 oxidation states.
Some other elements show +2 and +4 oxidation states even though they have electronic configurations other than f0, f7 or f14
For example: Sm2+ (4f6 ), Tm2+ (4f13), Pr4+ (4f1), Dy4+ (4f8). Nd4+ (4f2), etc. But these states are less stable than the +3 state which is characteristic of this family.
3) Ionization enthalpies
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.
The variation of third ionization enthalpies show some stabilities of empty (f0), half filled (f7) and completely filled (f14) levels. This is indicated by the abnormally low value of third ionization enthalpy of lanthanum (4f0), gadolinium (4f7) and lutetium (4f14).
The lanthanoids are silvery white but many of the trivalent ions of lanthanoids are coloured in the solid state as well as in solution.The colour of lanthanoid ions arises due to absorption in visible region of the spectrum resulting f-f transitions because they have partly filled orbitals. It has been observed that the colour of the ions containing nf-electrons is about the same as those with (14-n)f electrons.
|Ion||Electronic configuration||No. of electrons||Colour|
However, it has been observed that some di- and tetra-positive ions having same number of electrons (isoelectronic ions) as tripositive ions donot have same colour.
For example: Both Sm2+ and Eu3+ have same number of unpaired electrons (4f6) but they have different colours.
For example, Sm2+ is blood red while Eu3+ pale pink in colour.
5) Magnetic properties
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 electronic configurations are diamagnetic and all other trivalent lanthanoid ions are paramagnetic because they have unpaired electrons.