f block elements

    The f block elements appear in two series characterized by the filling of 4f and 5f orbitals in the respective third inner principal quantum level from outermost.
    The 4f series contains fourteen elements cerium to lutetium with the atomic number from 58 to 71 and are called Lanthanides as they appear after lanthanum.
    The 5f series contains fourteen elements thorium to lawrencium with the atomic number from 90 to 103 and are called actinides as they appear after actinium.

4f-block elements

    The 4f block elements have been variously called rare earth, lanthanoids, and lanthanum. The lanthanide atoms and their trivalent ions have the following general electronic configuration.
Lanthanide atoms
[Pd] 4fn 5S² 5P⁶ 5d¹ 6S²
where n has values 1 to 14
Lanthanide(M⁺³) ions
[Pd] 4fn 5S² 5P⁶
where n has values 1 to 14
f block elements electronic configuration
4f-block elements
    In 4f-block inner transition elements with increasing atomic number electrons are added to the deep-seated 4f orbitals. The outer electronic configuration of 4f-elements is 6S² and inner orbitals contain f -electrons.
    Only Cerium, Gadolinium, and Lutetium contain one electron in 5d orbital and the electronic configuration of the following elements are
Cerium
[Pd] 4f¹ 5S² 5P⁶ 5d¹ 6S²

Gadolinium
[Pd] 4f⁷ 5S² 5P⁶ 5d¹ 6S²

Lutetium
[Pd] 4f¹⁴ 5S² 5P⁶ 5d¹ 6S²
    Electrons of similar spin developed an exchange interaction which leads to the stabilization of the system. For the electrons of similar spin, repulsion is less by an amount called exchange energy.
    The greater the number of electrons with parallel spins the greater is exchanged interaction and the greater is the stability. This the basis of Hund's rules of maximum spin municipality.
    For the f subshell, maximum stability will result if there are seven electrons with parallel spins in the seven f orbitals, each orbitals having one when the f subshell is half-filled.
    Thus Gadolinium atom contains one electron in 5d orbital. 4f and 5d are very close in terms of energy levels. In such a case, the half-filled orbital is slightly more stable than orbital with one additional electron by increasing exchange energy. Thus when we add the next electron in half-filled 4f-orbital it may land on 5d-orbital.
    In the Lutetium atom, the maximum capacity of electrons in 4f orbitals is 14. Thus when we come from Ytterbium to Lutetium the next electron land on 5d orbital.

5f-block elements

    The 5f-block elements from thorium to lawrencium from the second series of inner transition elements are called man-made elements or Actinides. The Actinides atoms and their trivalent ions have the following general electronic configuration.
Actinides atoms
[Rn] 4fn 5d1-2 6S²
where n has values 1 to 14
Actinides(M⁺³) ions
[Rn] 4fn
where n has values 1 to 14
f block elements electronic configuration
5f-block elements


Questions answers

Question
    Why is the +3 oxidation state so common and stable in lanthanides?
    The nature of lanthanoids elements is such that three electrons are removed comparatively easy to give the normal trivalent state.
    The ground state electronic configuration of the natural lanthanoids atoms is
[Pd] 4fn 5S² 5P⁶ 5d¹ 6S²
    The electronic configuration of trivalent ions with the reducing three electrons is
[Pd] 4fn 5S² 5P⁶
where n is 0 to 14 from Lanthanum to Lutetium.
    As a consequence, the f electrons can not participate in the chemical reactions thus +3 oxidation state is common and stable in lanthanides.
Question
    Why does praseodymium possess electronic configuration 4f³ 6s² instead of the expected one 4f² 5d¹ 6s²?
Answer
    This can be explained by (n+l) rules, the orbital which has a higher value of (n+l) is the higher energy orbitals.
4f-orbital, (n+l) = 4+3 = 7
5d-orbital, (n+l) = 5+7 =7
    Thus for the above case which has the highest number of principal quantum number n is higher energy orbital. Thus 5d orbital is the higher energy orbitals.
    Again electrons are fed into orbitals in order of increasing energy until all the electrons have been accommodated.
    Thus praseodymium posses electronic configuration 4f³ 6s².

f block elements electronic configuration, electronic configuration of 4f and 5f-block elements and related questions answers

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