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Home » Class 12 » Chemistry » d and f Block Elements » Characteristics of Lanthanoids

Characteristics of Lanthanoids

Last Updated on February 16, 2023 By Mrs Shilpi Nagpal

Contents

  • 1 f-block elements
    • 1.1 (i) Lanthanoids
    • 1.2 (ii) Actinoids
  • 2 General Characteristics of Lanthanoids 
    • 2.1 1)  Electronic configuration
    • 2.2 2) Oxidation states
    • 2.3 3) Ionization enthalpies
    • 2.4 4) Colour
    • 2.5 5) Magnetic properties

f-block elements

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
(n-2)f1-14 (n-1)d0-1ns2

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.

(i) Lanthanoids

The series involving the filling of 4f-orbitals following lanthanum La (Z= 57) is called lanthanoid series. The elements present 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 are represented by the general symbol Ln. These occur very rarely and, therefore, these elements have also been called rare earth elements. 


(ii) Actinoids

The series involving the filling of 5f-orbitals is called actinoid series. It follows actinium, Ac (Z = 89) and the elements present in this series are called actinoids. They include the elements from thorium, Th (Z= 90) to lawrencium, Lw (Z=103). This series also consists of 14 elements.


General Characteristics of Lanthanoids 

The important general characteristics of lanthanoids are :


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
Neodymium Nd 60 [Xe] 4f45d06s2
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
In the succeeding 14 elements, 14 electrons are successively added to the 4f-subshell. The single 5d-electron shifts to the 4f-subshell. The single 5d-electron shifts to the 4f-subshell in all cases except gadolinium where such a shift gives the symmetry of half filled 4f-subshell and in lutetium where the 4f-subshell has already been completely filled. However, it has been observed that cerium also has 4f1 5d1 6s2 configuration.


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.

For example:


(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,
Ce4+= [Xe] 4f0
Tb4+ = [Xe] 4f7
(ii) Eu and Yb exhibit + 2 oxidation states. Europium and ytterbium get f7 and f14 configurations in +2 oxidation state
Eu2+ =  [Xe] 4f7
Yb2+= [Xe] 4f14

(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).

4) Colour

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
La3+ 4f0 0 Colourless
Lu3+ 4f14 14 colourless
Pr3+ 4f2 2 green
Tm3+ 4f12 12 green
Nd3+ 4f3 3 pink
Er3+ 4f11 11 pink
Sm3+ 4f5 5 yellow
Dy3+ 4f9 9 yellow
Eu3+ 4f6 6 pale pink
Tb3+ 4f8 8 pale pink

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.

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Filed Under: Chemistry, Class 12, d and f Block Elements

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