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General Characteristics Of Lanthanides

Chemical Bonding
These elements have an incomplete f-orbital of the anti-penultimate shell i.e. inner to the penultimate shell in addition to the incomplete d-orbital of the penultimate shell. F-block elements are also termed as inner transition elements as they are transition elements within the transition elements i.e. d-block elements.

Electronic configuration and series of f-block elements

An f-orbital can accommodate 14 electrons. This means f-block series can be said to include 14 elements. The general electronic configuration of the f-block elements is give as

(n-2) f1-14, (n-1) s2 (n-1 ) p6 (n-1 ) d10, ns2.

The f-block elements are grouped into two series basing on the nature of the f-orbital of the anti-penultimate shell (4f or 5f) into which the differentiating electron enters.

Lanthanides

The f-block series in which the differentiating electron enters in to the 4f orbital of the anti-penultimate shell are termed as Lanthanides. This series starts from Lanthanum whose atomic number is 57 and continues up to the next 14 elements i.e. 71 which is Lu (Lutetium). It should be noted that all the elements of the lanthanide series (named after the first element of the series Lanthanum) resemble one another very closely due to the presence of same number of electrons in outermost and the penultimate shells.

General Characteristics:

Though Lanthanum resembles the members of the f-block elements, it is in fact studied under d-block elements as the filling of the electrons into the f-orbital begins from cerium. Thus, the 4f-block elements start from Cerium and continue until Lutetium and as they are following lanthanum they are called as lanthanides or lanthanones.
  • Electronic Configuration of Lanthanides: as the 4f and 5d electrons are so close in energy it is not possible to decide whether the electron has entered the 5d or 4f orbital. However, it is considered that the 5d orbital remains vacant and the electrons enter into the 4f orbital except for gadolinium, Gd (Z=64) where the electron enters into the 5d orbital due to the presence of half filled d-orbital. At Ytterbium (z=70) all the 4f orbital's are completely filled and hence, the differentiating electron of the next element that is lutetium (z=71) enters in to the 5d orbital.

  • The complete electronic configuration of Lanthanides can be given as

    1s2 2s2p6 3s2p6d10 4s2p6d10f0-14 5s2p6 d0-1 6s2

  • Oxidation States: Lanthanides show variable oxidation states but the degree of variability is less compared to the transition elements. The most stable oxidation state of Lanthanides is +3. In addition to the most stable +2 state, Lanthanides also show +2 and +4 oxidation states. These additional oxidations states also show stability due to presence of either half filled or completely filled or empty 4f subshell. It should be noted that the +2 and +4 oxidation states are unstable in aqueous solutions except for Ce+4 salts such as the ceric sulphate which acts as an oxidising agent in volumetric analysis.

  • Ionic Radii and Lanthanide contraction: the atomic size decreases with the increase in atomic number as we move across from La to Lu. Thus, among lanthanides, lanthanum has the largest atomic radius and lutetium has the smallest atomic radius. This gradual decrease in the size of an atom is said as lanthanide contraction.

    The major cause for lanthanide contraction is due to the inappropriate shielding of the 4f electrons. This is due to the improper shape of the f-orbitals. Thus, with the increase in the atomic number, the incoming electrons increase in number and hence, the nuclear charge. Due to inappropriate shielding, the electrons get attracted towards the nucleus resulting in the decrease of the size. However, the decrease in size is not uniform throughout the lanthanides. A rapid decrease is seen in first six elements compared to the rest of the elements.

  • Color: All lanthanide metals are silver white. The trivalent lanthanide ions are colored both in solid state and in aqueous solution. It should be noted that the color change is seen only in case of cations. The color of a cation depends on the number of unpaired f electrons. Lanthanides with either half-filled or completely filled orbitals are colorless.

    The difference in the color is due to differential absorption of certain wavelengths in visible region. The absorption depends on the energy needed by the electrons to be excited to the next higher levels and the availability of orbital's to which 4f electrons can be excited.

  • Magnetic Properties: Elements with paired electrons does not show any magnetism due to cancellation of the opposite spins due to pairing. The unpaired electrons show paramagnetic nature to some extent. Thus lanthanum and lutetium with no unpaired electrons does not show magnetism and are said to be diamagnetic in nature. The remaining members of lanthanide series are tripositive and are paramagnetic in nature.

  • Complex formation: The large size of lanthanides imparts a very low charge density to them as a result they cannot cause much polarization and hence, do not show any tendency to form complexes. However, they form complexes with few chelating agents such as EDTA.

    It should be noted that in the lanthanide series the tendency to form complexes and the stability of the complexes increases with the increase in atomic number. This property can best be utilized in the separation of the lanthanides. In case of the hydrated ions i.e. lanthanides in aqueous solutions have low tendency to form complexes and the ability to form complexes decreases with the increasing atomic number. The order of complex formation can be best represented as

    Ln4+ > Ln3+ > Ln2+

  • Reactivity: as all the lanthanides show a similar electronic configuration and the +3 oxidation states, they show similarity in their properties. The degree of similarity in the reactivity of the lanthanides is greater than that of the transition elements as the unpaired electrons present in the inner 4f-orbital are well shielded by the outer 5s, 5p, and 5d orbital's. The size of all the lanthanides is also nearly similar due to lanthanide contraction. Due to the above reasons the size of the ions is almost similar and hence, they show great similarity in their chemical properties.

    Lanthanides show very high ionization energies and electro negativity. The ionization energies of the lanthanides are comparable to those of the alkaline earth metals and are hence, said as highly reactive elements.

    • All lanthanides react readily upon exposure to air and tarnish.

    • They readily dissolve in hot water liberating hydrogen. They can also dissolve in cold water.

    • They react with nitrogen and hydrogen forming the corresponding nitrides and hydrides.

    • Lanthanides react with other non-metals such as halogens, sulphur, phosphorus, carbon and silicon and form corresponding compounds.

    • The high oxidation potentials indicate their strong electro positive nature to act as strong reducing agents.

    • Lanthanides react with acids and liberate hydrogen.



Solved problems
  • Which of the following is true?

    • Lanthanides are comparable to alkaline earth metals in reactivity.
    • Lanthanides tarnish upon exposure to air.
    • The +3 oxidation state is commonly seen among lanthanides.
    • All the above.
    Answer: d
  • Number of elements in the lanthanide series?

    • 14 elements
    • 20
    • 5
    • 16
    Answer: a
  • The decrease in size of the lanthanides with the increase in atomic number is due to

    • Lanthanide contraction
    • Loss of electrons
    • Gain of electrons
    • Size does not decrease.
    Answer: a

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