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CBSE Class 12th Chemistry Notes: The d- and f-Block Elements (Part-II)

Oct 10, 2016 15:00 IST

    This article provides you the revision notes on Class 12 Chemistry: Chapter- The d- and f-Block Elements, to give you a quick glance of the chapter. This article is a continuation of the revision notes on Class 12 Chemistry, Chapter- The d- and f-Block Elements, Part-I. In Part-I you discovered about the d-block elements and their important compounds. In Part-II, you will get acquainted with the f-block elements. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.

    The main topics covered in this part are:

    • f –Block elements or inner transition elements
    • Lanthenoids 
      • Definition
      • General properties
      • Uses
    • Definition
      • General properties
      • Actinoids

    The key notes of the chapter are as follows:

    f –Block elements or Inner Transition elements

    The elements in which the differentiating electron enters the penultimate energy level i.e. (n−2)f, are called f-block elements. Due to such electronic configuration where the last electron enters the 4f or 5f orbitals that are lower than the outermost electrons, f-block elements are also named as inner transition elements.

    Depending upon the fact whether the last electron enters the 4f or 5f-orbitals, f-block elements are differentiated into lanthanoids and actinoids.

    1. Lanthanoids: The 14 elements immediately following lanthanum, i.e., Cerium (58) to Lutetium (71) are called lanthanoids. They belong to first inner transition series. Lanthanum (57) has similar properties.

    General properties of lanthanoids:

    • Electronic configuration: The general electronic configuration of the lanthanoids is
    • Atomic and ionic Sizes: The decrease in in atomic and ionic radii from lanthanum to lutetium is not quite regular but there is a regularity in the size of M3+ ions. The regular decrease in size of M3+ ion is attributed to the imperfect shielding of one electron by another in the same 4f subshell. This regular decrease in size amongst lanthanides as atomic number increases is known as the lanthanoid contraction.
    • Formation of coloured ions: Lanthanide form ions which are coloured in both solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of f-electron.

    Exception: Lu3+ ions do not show any colour due to the absence of any unpaired electron in the 4f subshell which is fully filled.

    • Magnetic character: Lanthanide ions also show paramagnetism. In lanthanides, the magnetic moment is due to both spin magnetic moment as well as orbital magnetic moment.
    • Oxidation state: The most common oxidation state of lanthanides is + 3 which is obtained by using two electrons in 6s and one electron from 5d subshell.

    Exception: Some elements show +2 and +4 oxidation states. This irregularity arises mainly from the extra stability of empty, half-filled or filled f subshell.

    For example:

                   

    Physical properties of lanthanoids:

    • All the lanthanoids are silvery white soft metals and tarnish rapidly in air, the hardness increases with increasing atomic number.
    • The melting points range between 1000 to 1200 K but samarium melts at 1623 K.
    • They are also good conductors of heat and electricity.

    Chemical properties of lanthanoids:

    Some important chemical reactions of lanthenoids are:

                                             

    Formation of alloys: Lanthanoids are all used in steel industry for making alloy steels. Important and well-known alloy is misch-metal and it con-sists of lanthanoid (90-95%), iron (4-5%) and trace amount of S, C Ca and Al.

    Uses:

    (i) Misch-metal is used in making tracer bullets, shell and lighter flint.

    (ii) Mixed oxides of lanthanoids are used as a catalyst in petroleum cracking. Some individual oxides of lanthanoids are used as phosphors in television screens and similar fluorescing surface.

    2. Actinoids:  The 14 elements immediately following actinium (89), with atomic numbers 90 (Thorium) to 103 (Lawrencium) are called actinoids. They belong to second inner transition series. In actinoids the filling of electrons takes place in the anti-penultimate  subshell.

    General properties of actinoids:

    • Electronic configuration: The general electronic configurations for the actinoids is [Rn]5f1−146d0−17s2 , where Rn is the electronic configuration of the element Radium. The fourteen electrons are formally added to  though not in Thorium  but onwards from it and the 5f subshell is complete at Lr (Z = 103). The irregularities in the electronic con-figuration of the actinoids are releated to the stabilities of  of empty, half-filled or filled f subshell.
    • Oxidation state: The dominant oxidation state of actinoids is +3. However they also show variable oxidation states due to the comparable energy of 5f, 6d and 7s subshells.For example: The uranium shows oxidation states of and. The element neptunium (Z = 93) show an oxidation state upto +7.
    • Magnetic character: Actinoids also show paramagnetism but their magnetic properties are much more complex than those of the lanthanoids.
    • Atomic and ionic sizes: In actinides, the ionic radii decreases as we move down the series. This decrease in ionic radius is termed as actinide contraction. This effect is due to poor screening offered by 5f electrons.
    • Ionisation enthalpy: The ionisation enthalpies of actinoids are lower than those of corresponding lanthanoids. This is because the orbitals in actinoids penetrate less into the inner core of electrons, and the  electrons are more effectively shielded from the nuclear charge than are the electrons of the corresponding lanthanoids. As the outer electrons are less tightly held, less amount of energy is required to ionise an atom.
    • Metallic character: The actinoids are all metals with silvery appearance. These metals are highly reactive when finely divided.

    Chemical properties:

    • They react with boiling water to give a mixture of oxide and hydride.
    • All these metals are attacked by HCl but slightly affected by HNO3 due to the formation of a protective oxide layer on their surface.
    • They combine with most of the non-metals at moderate temperature.

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