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Lanthanides and Actinides

All known elements are arranged in the long form of the periodic table. It was proposed by Mendeleev’s and also called as a Modern periodic table. The long form of the periodic table was based on the increasing order of atomic number of elements. It is a tabular form in which elements are arranged in columns and rows. The columns of the periodic table are called as groups whereas the horizontal rows are known as periods. The entire table is composed of 18 groups and 7 periods. There are two series are the bottom of main table which consists of 14 elements in each series.

Complete table can be classified as s, p, d and f-block elements. First and 2nd group belong to s-block of the periodic table as their valence electrons are filled in the s-orbital of the atom. Similarly, from 3rd to 12th group, elements are related to d-block and also known as the transition elements. The p-block of the periodic table is comprised from 13th to 18th group of the periodic table.

All metals are placed on the left side of the periodic table. Around 75% of elements are metallic in nature and belong to s, d and f-blocks. The non-metals and metalloids are placed in p-block with few metals. The elements of f-block are also called as Lanthanides and Actinides. These names come from the Lanthanum and Actium after which, both series of f-block are started.

 

Lanthanide and Actinide Series

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The f-block elements in which the last electron or the differentiating electron enters in (n-2) f-orbitals and the general electronic configuration is (n-2)f1-14 (n-1)d0-1 ns2 consists of two series of inner transition elements-

Lanthanides (the fourteen element following lanthanum)

The elements in which the last electron enters one of the 4f-orbitals are called 4f-block elements or first inner transition series. They are also called Lanthanides (or) lanthanones because they come immediately after lanthanum. 

Actinides (the fourteen element following actinium)

The elements in which the last electron enters one of the 5f-orbitals are called 4f-block elements or second inner transition series. They are also called Actinides because they come immediately after actinium. The general electronic configuration is [Rn]5f1-14 6d0-1 7s2

Lanthanides (58Ce- 71Lu) - This series starts from Lanthanum ( atomic numbe r= 57) and continues up to Lu (Lutetium) as shown in the table above. All the elements of the lanthanide series resemble each other very closely due to the presence of the same number of electrons in the outermost and the penultimate shells. They are also called rare earth elements. Though Lanthanum is a d-block element it is included in the lanthanides series as it resembles them.

General Characteristics

Some important characteristics are given below-

  1. Electronic Configuration of Lanthanides: The general electronic configuration is. All have electronic configuration with 6s2 is common but variable occupancy of 4f and 5d-subshells due to the closeness in energy of 4f and 5d electrons. So it is considered that the 5d orbital remains vacant and the electrons enter into the 4f orbital. Exception are in the case of gadolinium, Gd (Z = 64) where the electron enters the 5d orbital due to the presence of half filled d-orbital and in Ytterbium (z = 70) in which all the 4f orbital's are completely filled and the differentiating electron of the next element that is lutetium (z = 71) enters the 5d orbital. The complete electronic configuration of Lanthanides can be given as 1s2 2s2 p6 3s2p6d104s2p6d10f0-14 5s2p6 d0-1 6s2.
  2. Oxidation States: Lanthanides show variable oxidation states. The most stable oxidation state of Lanthanides is +3. They also show +2 and +4 oxidation states due to the presence of either half filled or completely filled or empty 4f sub shell.
  3. Color: Many of lanthanide metals are silver white. The lanthanide ions with +3 oxidation state are colored both in solid state and in aqueous solution. The color of a cation depends on the number of unpaired f electrons.
  4. Magnetic Properties: The lanthanide ions other than f0 and f14 type are paramagnetic in nature due to unpaired electrons in f-orbitals.
  5. Melting and boiling point: They have fairly high melting point but there is no definite trend in the melting and boiling point of lanthanides.
  6. Density: They have high density ranging between 6.77 to 9.74 g cm-3. Its increases with increasing atomic number.
  7. Ionization enthalpies: They have low ionization enthalpy.
  8. Complex formation: They don't have much tendency to form complexes because of low charge density. The order of complex formation can be best represented as Ln4+ > Ln3+ > Ln2+.
  9. Reactivity: All the lanthanides show the same electronic configuration and the +3 oxidation states, they show similarity in the reactivity which is greater than the transition elements. This is due to shielding of the unpaired electrons of the inner 4f-orbital by the outer 5s, 5p, and 5d orbital's. Due to the small change in the size of the ions, they show great similarity in their chemical properties. The first few members are quite reactive. A few properties are given below.

  • All lanthanides react rapidly upon exposure to air.
  • They dissolve in hot water and react with acid, liberating hydrogen.
  • They act as a strong reducing agent because of the strong electro positive nature
  • They form the nitrides and hydrides after reacting with nitrogen and hydrogen respectively.
  • They also react with non-metals like halogens, sulfur, phosphorus, carbon and silicon and form their corresponding compounds.

The atomic size or ionic radii of tri positive lanthanide ions show a steady and gradual decrease with the increase in atomic number from La to Lu. Although they show some irregularities, the ionic radii decrease steadily from La to Lu. This gradual decrease in the size with increasing atomic number is called lanthanide contraction.

Cause of lanthanide contraction

The major cause of lanthanide contraction is due to the inappropriate shielding of the 4f electrons due to the improper shape of the f-orbitals. As the atomic number increases in the lanthanide series, for every proton in the nucleus the extra electron goes to fill the 4f-orbitals.

The 4f-electrons constitute inner shells and are rather ineffective in screening the nuclear charge. Thus, there is a gradual increase in the effective nuclear charge experienced by the outer electrons. So the attraction of the nucleus for the electrons in the outermost shell increases as the atomic number increases and the electron cloud shrinks.

This results in gradual decrease in the size of lanthanides with increasing atomic number. The decrease in size is not regular throughout the lanthanides. A rapid decrease is seen only in the first six elements compared to the rest of the elements.

Consequences of lanthanide contraction

  1. Similarity of second and third transition series- Generally the size of atoms increases down a group. So the size of the atoms of an element of second transition series is larger than an atom of the first transition series but the size of the atom of third transition series is nearly same as that of the atom of the second transition series. The similarity in the size of atoms of the elements belonging to the same group of the second and third transition series is due to the effect of lanthanide contraction. For example- radius of Zr radius of Hf & radius of Nb radius of Ta etc.
  2. Difficulty in the separation of lanthanides- As there is small change in the ionic radii of lanthanides so their chemical properties are similar. This makes the separation of elements in the pure state difficult. This contraction makes the slight difference in size so the properties like solubility, complex formation, hydration etc shows some differences and its possible to separate them by ion exchange methods.
  3. Effect on the basic strength of hydroxides- As the size of lanthanides decreases from La to Lu, the covalent character of the hydroxides increases and hence their basic strength decreases. Thus, La(OH)3 is more basic and Lu(OH)3 is least basic.
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Actinides

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Actinides are termed as the second inner transition series. This series is named after actinium, the first element of the series. Only the first four elements occur in nature. The other elements are made by nuclear bombardment because most of these are radioactive.

General characteristics of Actinides

Some general characteristic of actinides is given below.
  1. Electronic Configuration: All the actinides have common 7s2 configuration and variable occupancy of 5f and 6d sub shells. The general electronic configuration of actinium is [Rn] 5f0-146d0-2 7s2 where, Rn is the electronic configuration of the element Radium. The irregularities in the electronic configuration are related to the stabilities of half filled or full filled orbitals (f0, f7, f14 ).
  2. Oxidation States: Unlike lanthanides, actinides show a variety of oxidation states from +3 to +6 due to the very small energy gap between 5f, 6d and 7s sub shells. The principal oxidation states are +3 and +4. The +3 oxidation state is the most stable. The +4 oxidation state is the most stable in Th and Pu. +5 in Pa and Np and +6 is seen in U. In actinides, the distributions of oxidation states are uneven.
  3. Complex formation: The degree of complex formation decreases in the order. M4+ > MO2+2 > MO+2 where, M is an element of the actinide series. There is a high concentration of charge on the metal atom.
  4. Melting and boiling point: They have high melting and boiling points like lanthanides but don’t show any regular trend with increasing atomic number.
  5. Density: All actinides except thorium and amercium have high density.
  6. Electropositive character: These metals are highly electropositive in nature like lanthanides.
  7. Ionization enthalpies: The actinides have lower ionization enthalpies than lanthanides because 5f is more effectively shielded from nuclear charge than 4f.
  8. Magnetic behavior: All actinides are paramagnetic in nature which depends on the presence of unpaired electrons.
  9. Radioactivity: All are radioactive in nature. First few members have relatively long half lives but the remaining have half-lives from a few days to a few minutes.
  10. Color of the ions: Actinides ions are generally colored due to f – f transitions. It depends upon the number of electrons in 5f orbitals.
  11. Chemical behavior: They are highly reactive metals in fine state and a strong reducing agent. A few properties are given below.
  • They react with boiling water to give oxide and hydride.
  • They combine with most of the non-metals at moderate temperature.
  • All these metals are attacked by HCl acid but the effect of nitric acid is very small.
Actinide Contraction

The size of atoms or M+3 ions decreases regularly along the actinides series with increase in atomic number from Th to Lr. The steady decrease in ionic radii with an increase in atomic number is referred to as actinide contraction just like lanthanide contraction. 

The reason for this contraction is the poor shielding effect of 5f electron by another in the same shell which increases the effective nuclear charge and thus contraction in size of the electron cloud. This is because 5f orbitals extend in space beyond 6s and 6p orbitals whereas, 4f orbitals are buried deep inside the atom.

Similarities Between Lanthanides and Actinides

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  1. Both lanthanides and actinides involve the filling of f-orbitals.
  2. Both exhibit a common oxidation state of +3.
  3. Both are electropositive and very reactive.
  4. Both exhibit magnetic and spectral properties.
  5. Lanthanides exhibit lanthanide contraction and actinides exhibit actinide contraction.

Difference Between Lanthanides and Actinides

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Lanthanides Actinides
They have the ability to show a maximum oxidation state of + 4 Actinides show variable oxidation states of + 3, + 4, + 5, + 6 and + 7.
They have smaller tendency to form complexes. They have a good tendency to form complexes with ligands such as thio-ethers.
All lanthanides are non-radioactive except promethium. They are radioactive in nature.
They do not form oxo-ions Actinides form oxo-ions such as UO+, NpO2+.
They are non radioactive in nature. Actinides are radioactive.

Uses of Lanthanides and Actinides

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The important uses of lanthanides are given below.
  1. They don’t find any use in their pure state. They are used in the production of alloys of steel to improve the strength and workability of steel. A well known alloy is mischmetal which is used in making magnesium based alloy (Mg mixed with 3% of mischmetal). This is a pyrophoric alloy which is used in making bullets, shells and lighter flints.
  2. Their oxides are used in the glass industry for polishing glass and making colored glass for goggles and television screens.
  3. Mixed oxides of lanthanides are used in petroleum cracking.
  4. Due to their paramagnetic and ferromagnetic nature, they are used in magnetic and electronic devices.
  5. Ceric sulphate is a well known oxidizing agent which is used in volumetric analysis.

Uses of Actinides

The most useful actinides are thorium, uranium and plutonium.
  1. Thorium is used in the atomic reactors and in the treatment of cancer. Their salts are used in making incandescent gas mantles.
  2. Uranium is used as nuclear fuel. Their salts are used in the glass industry for imparting green color, ceramic industry, textile industry and in medicines too.
  3. Plutonium is also used as nuclear fuel and for making atomic bombs.
More topics in Lanthanides and Actinides
Lanthanide Contraction
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