The Inner Transition f-Block Elements

 The Inner Transition f-Block Elements

   ·      28 elements from atomic number 58(Ce) to 71(Lu) —14 elements and from atomic number   90(Th)  to 103(Lr)  —14 elements . these have been arranged in two horizontal rows below the periodic table. These elements are collectively called f- block elements

     Since the last electron enters one of the f-orbitals

    ·      These are also known as inner transition series

    ·      f-block consists of two series of elements known as Lanthanides or Lanthanons and Actinides or Actinons.

     ·      The lanthanide series follows lanthanum (At. No. 57), because lanthanum closely resembles the lanthanoids

·            Position in periodic table -the 14 members of lanthanide series have been placed along with lanthanum in the third group and sixth period and similarly 14 members of the actinide series have been placed with actinium in the third group and seventh period

    ·        The justification for assigning one place to these elements has been given on the basis of their similar properties. The properties are so similar that the fifteen elements from La to Lu can be considered as equivalent to one element. The same explanation can be given in the case of actinides.

    ·      In case, these elements are assigned different positions in order of their increasing atomic numbers, the symmetry of the whole arrangement would be disrupted. Due to this reason, the two series of elements, i.e., lanthanides and actinides are placed at the bottom of the periodic table

    ·      4f- series( lanthanide series) - There are fourteen elements from cerium (At. No. 58) to lutetium (At. No. 71) in this series. 4f-orbitals are gradually filled up. In past, elements were called rare earths.

    ·      5f-series (Actinides series): There are fourteen elements from thorium (At. No. 90) to lawrencium (At. No. 103) in this series. 5f-orbitals are gradually filled up.

   The elements from atomic number of 93 onwards are called transuranic or trans-uranium elements

 

GENERAL CHARACTERISTICS OF LANTHANIDES

 Electronic configuration –

     ·      General Electronic configuration -       (n-2)f^{1 -14}(n-1)d^0-1ns²     or   4f^1-14 5d^0-1 6s² 

     ·      The energies of 5d- and 4f-orbitals are nearly similar and thus their fillings show certain irregularities.

    

·        All tri-positive ions (the most stable oxidation state of all lanthanoids) have 4f^1-14 electronic configuration  

Atomic size and ionic size

     ·      There is decrease in atomic and ionic radii from lanthanum to lutetium due to lanthanoid contraction. However, the shielding of one 4f electron by another is less than a d-electron by another with the increase in nuclear charge along the series.

     ·      The decrease in atomic radii is quite not regular as it is regular in M³⁺

    Lanthanoid contraction –

     is a regular decrease in atomic radii by filling of 4f before 5d orbital. Since 4f orbital causes poor shielding effect(screening) than 5d.

    Consequences of lanthanoid contraction –

    ·      Similarity in atomic size of 2^nd and 3^rd  transition series.

    ·      Variation in basic strength of hydroxide

       Basic strength decreases from La(OH)₃ to Lu(OH)₃ due to lanthanoid contraction ,size of M³⁺ decreases its result is the increase in covalent nature

    Oxidation states

    ·      common oxidation à +3

    ·      In   La, Gd, Lu are especially stable because M³⁺, ions have stable electronic configuration as 4f⁰, 4f⁷ ,4f¹⁴ respectively

    ·      Ce(4f⁰ )and Tb(4f⁷ )exhibit à +4  , also show+3

    ·      Lanthanoids have a tendency to attain the oxidation state +3 due to most stable.

    ·      Ce⁴⁺ is good oxidising agent  while Sm⁺² is good reducing agent

    Ce⁴⁺  +  Fe²⁺------------> Ce³⁺  +  Fe³⁺

    2 Sm²⁺  +  H₂O -------> 2Sm³⁺  +  2OH^-  + H₂

    ^·              Eu²⁺ is strong reducing reagent while Tb⁺⁴ is an oxidant

    ·      The E° Value for Ce4+/Ce3+ is +1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent.       

    Colour

     The colour is due to f-f transitions since they have partly filled f-orbitals

        Mischmetal consists of a lanthanoid metal (nearly 95%) and iron (nearly 5%) and traces of S,C, Ca, and Al . these are used in Mg-based alloy to produce bullets, shell and light flint.