Characteristics of Lanthanoids - Chemistry, Class 12, d and f Block Elements

Contents

1 f-block elements
- 1.1 (i) Lanthanoids
- 1.2 (ii) Actinoids
2 General Characteristics of Lanthanoids

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)f^1-14 (n-1)d^0-1ns²

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] 5d¹6s²
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.