Lanthanides and Actinides

 

Table of Content

Lanthanides Electronic Configuration Atomic and Ionic Sizes of Lanthanides Consequence of Lanthanide Contraction Solved Problem Complex formation ​Actinides Electronic Configuration Melting and boiling point Related Resources

Lanthanides

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

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 3^rd transition series are very similar to those of the corresponding elements of the 2^nd  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] 5f^0-146d^0-2 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.
   

Related Resources

• Download the  past year papers of IIT JEE  

• Have a look at the  syllabus of chemistry for  IIT JEE