The lanthanides and the actinides make up the f block of the periodic table. The lanthanides are the elements produced as the 4f sub level is filled with electrons and the actinides are formed while filling the 5f sub level

anthanide and Actinide Series

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)f^1-14 (n-1)d^0-1 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.
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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]5f^1-14 6d^0-1 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 3s²p⁶d¹⁰4s²p⁶d¹⁰f^0-14 5s²p⁶ d^0-1 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 for 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.

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.