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Lanthanides and Actinides

Lanthanides

Lanthanides consist of elements that follow lanthanum and involve the filling of 4f subshell 

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Electronic Configuration

[Xe] 4fn+1 5d° 6s2 or [Xe] 4fn 5d1 6s2

The general valence shell electronic configuration of lanthanides is 4f1−146s2.

Electronic configurations of lanthanum and lanthanides are listed in the table

Atomic Number

Name

Symbol

Electronic configurations

Radii/pm

Ln

Ln2+

Ln3+

Ln4+

Ln

Ln3+

57

Lanthanum

La

5d16s2

5d1

4f0

-

187

106

58

Cerium

Ce

4f15d16s2

4f2

4f1

4f0

183

103

59

Praseodymium

Pr

4f36s2

4f3

4f2

4f1

182

101

60

Neodymium

Nd

4f46s2

4f4

4f3

4f2

181

99

61

Promethium

Pm

4f56s2

4f5

4f4

-

181

98

62

Samarium

Sm

4f66s2

4f6

4f5

-

180

96

63

Europium

Eu

4f76s2

4f7

4f6

-

199

95

64

Gadolinium

Gd

4f75d16s2

4f35d1

4f7

-

180

94

65

Terbium

Tb

4f96s2

4f9

4f8

4f7

178

92

66

Dysprosium

Dy

4f106s2

4f10

4f9

4f8

177

91

67

Holmium

Ho

4f116s2

4f11

4f10

-

176

89

68

Erbium

Er

4f126s2

4f12

4f11

-

175

88

69

Thulium

Tm

4f136s2

4f13

4f12

-

174

87

70

Ytterbium

Yb

4f146s2

4f14

4f13

-

173

86

71

Lutetium

Lu

4f145d16s2

4f145d1

4f14

-

-

-

Atomic and Ionic Sizes of Lanthanides

  • Atomic and ionic radii of lanthanides decrease with an increase in atomic number. This gradual decrease is known as lanthanides contraction.

  • Because of the lanthanides contraction, the radii of the elements of the 3rd transition series are very similar to those of the corresponding elements of the 2nd  transition series elements.

Oxidation States of Lanthanides

Lanthanides exhibit the oxidation state of +3. Some of them also exhibit the oxidation state of +2 and +4.

  • a noble gas configuration e.g. Ce4+ (f0)

  • a half filled f shell e.g. Eu2+ (f7)

  • a completely filled f shell e.g. YB22+ (f14

Lanthanide contraction

It is observed that in lanthanide series, there is a progressive decrease in the atomic and  ionic radii with increasing atomic number . This regular decrease with increase in atomic number is called lanthanide contraction. This is due to the weak shielding of f orbitals. These f orbitals are unable to counter balance the effect of increasing nuclear charge because of which the size keeps on decreasing with increase in atomic umber. 

Causes of Lanthanide Contraction:  

As we move along the period from left to right in lanthanide series, the atomic number increases i.e. number of protons keeps on increasing.For every proton added in the nucleus the extra electron goes to the same 4f orbital.
The 4f orbital shows poor shielding effect because of which there is a gradual increase in the effective nuclear charge experienced by the outer electrons. Thus, the attraction of the nucleus for the electrons in the outermost shell increases in atomic number.

Consequence of Lanthanide Contraction

  • Separation of Lanthanides: Without lanthanide contraction all the lanthanides would have same size because of which if would have been very difficult to separate them but due to lanthanide contraction their properties slightly vary. The variation in the properties is utilized for separating them.

  • Basic Strength of Hydroxide: Because of the lanthanide contraction, size of M3+ ions decreases and there is increase in covalent character in M–OH and hence basic character decreases.

  • Similarity of 2nd and 3rd transition series i.e. 3d and 4 d series: The atomic sizes of second row transition elements and third row transition elements are almost similar. This is also an effect of lanthanide contraction. As we move down the from form 4d to 5d series, the size must increase but it remains almost same due to the fact that the 4f electrons present in the 5d elements show poor shielding effect. 

Refer to the following video for f block elements

Solved Problem

Question         

Why Sm2+, Eu2+ and Yb2+ ions in solutions are good reducing agents but an aqueous solution of Ce4+ is a good oxidizing agent?

Solution:

+3 state is the most stable oxidation state of lanthanides. Thus the  ions in +2 oxidation state tend to change to +3 oxidation state by loss of electron and those ions which are  in +4 oxidation state tend to change to +3 oxidation state by gain of electron.

Complex formation   

  • The lanthanides do not show much tendency to form complexes due to low charge density because of their size. However, the tendency to form complex and their stability increases with increasing atomic number.

Chemical Behaviour

The first few members of the series are quite reactive like calcium. However with increasing atomic number, their behaviour becomes similar to that of aluminum. 

  • Lanthanides combine with hydrogen on gentle heating. When they are heated with carbon result in formation of  carbides. On burning in the presence of halogens, lanthanides form halides.

  • Lanthanides react with dilute acids to liberate hydrogen gas.

  • Lanthanides form oxides and hydroxides of the type N2O3 and M(OH)3 which are basic alkaline earth metal oxides and hydroxides.

Uses of Lanthanides

  • Lanthanide are used in  the production of alloy steels for plates and pipes. 

  • Mixed oxides of lanthanides are used as catalysts in petroleum cracking industries.

  • Some lanthanum oxides are used as phosphors in television screens and other fluorescing surfaces.

​Actinides

Actinides consist of elements that follow actinium and involve the filling of 5f subshell . 

Electronic Configuration

[Rn] 5f0-146d0-2 7s2

Oxidation States

The dominant oxidation state of these elements is +3 (similar to lanthanides). Besides +3 state, they also exhibit +4 oxidation state. Some actinides show still higher oxidation states. The maximum oxidation state first increases upto the middle of the series and then decreases i.e. it increases from +4 for Th to +5, +6 and +7 for Pa, V and Np but decreases in the succeeding elements.

Melting and boiling point:  

They have high melting and boiling points like lanthanides but don’t show any regular trend with increasing atomic number.

Density: 

All actinides except thorium and amercium have high density.

Ionization enthalpies:

The actinides have lower ionization enthalpies as comapre to lanthanides because 5f is more effectively shielded from nuclear charge than 4f.

Magnetic behavior:

All actinides are paramagnetic in nature. The paramagnetic nature which depends on the presence of unpaired electrons.

Radioactivity:

All the actinides are radioactive in nature.  Radioactivity increases with increase in atomic number. 

Chemical Behaviour

The ability of actinides to exist in different oxidation states has made their chemistry more complex. Moreover, most of these elements are radioactive and the study of their chemistry in the laboratory is difficult.

  • They react with boiling water to give a mixture of oxide and hydride.

  • The combine with most of the non – metals at moderate temperature.

  • All these metals are attacked by HCl but the effect of HNO3 is very small due to the formation of a protective oxide layer on their surface.

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