Lanthanides

Lanthanides and actinides are collectively called f-block elements because last electron in them enters into f– orbitals of the antepenultimate (i.e., inner to penultimate) shell partly but incompletely filled in their elementary or ionic states. The name inner transition, elements is also given to them because they constitute transition series with in transition series (d-block elements) and the last electron enters into antepenultimate shell (n-2) f. In addition to incomplete d-subshell, their f-subshell is also incomplete. Thus, these elements have three incomplete outer shells i.e., (n–2), (n–1) and n shells and the general electronic configuration of f-block elements is (n–2) f^1–14 (n–1)d^0–10ns².

(1)     Lanthanides : The elements with atomic numbers 58 to 71 i.e. cerium to lutetium (which come immediately after lanthanum Z = 57) are called lanthanides or lanthanones or rare earths. These elements involve the filling of 4 f-orbitals. Their general electronic configuration is, [Xe]4f^1–14 5d^0–10 6s². Promethium (Pm), atomic number 61 is the only synthetic (man made) radioactive lanthanide.

 

Properties of lanthanides

(i)      These are highly dense metals and possess high melting points.

(ii)     They form alloys easily with other metals especially iron. e.g. misch metal consists of a rare earth element (94–95%), iron (upto 5%) and traces of S, C, Ca and Al, pyrophoric alloys contain Ce (40–5%), La + neodymium (44%), Fe (4–5%), Al (0–5%) and the rest is Ca, Si and C. It is used in the preparation of ignition devices e.g., trace bullets and shells and flints for lighters.

(iii)    Oxidation state : Most stable oxidation state of lanthanides is +3. Oxidation states +2 and +4 also exist but they revert to +3 e.g. Sm²⁺, Eu²⁺, Yb²⁺ lose electron to become +3 and hence are good reducing agents, where as Ce⁴⁺, Pr⁴⁺, Tb⁴⁺ in aqueous solution gain electron to become +3 and hence are good oxidizing agents. There is a large gap in energy of 4 f and 5d subshells and thus the number of oxidation states is limited.

(iv)    Colour : Most of the trivalent lanthanide ions are coloured both in the solid state and in aqueous solution. This is due to the partly filled f-orbitals which permit f–f transition. The elements with xf electrons have a similar colour to those of (14 – x) electrons.

(v)     Magnetic properties :  All lanthanide ions with the exception of Lu³⁺, Yb³⁺ and Ce⁴⁺ are paramagnetic because they contain unpaired electrons in the 4f orbitals. These elements differ from the transition elements in that their magnetic moments do not obey the simple “spin only” formula µ_eff =  B.M. where n is equal to the number of unpaired electrons. In transition elements, the orbital contribution of the electron towards magnetic moment is usually quenched by interaction with electric fields of the environment but in case of lanthanides the 4f-orbitals lie too deep in the atom for such quenching to occur. Therefore, magnetic moments of lanthanides are calculated by taking into consideration spin as well as orbital contributions and a more complex formula

                    µ_eff =  B.M.

Which involves the orbital quantum number L and spin quantum number S.

(vi)    Complex formation : Although the lanthanide ions have a high charge (+3) yet the size of their ions is very large yielding small charge to size ratio i.e., low charge density. As a consequence, they have poor tendency to form complexes. They form complexes mainly with strong chelating agents such as EDTA, β-diketones, oxine etc. No complexes with π-bonding ligands are known.

(vii)   Lanthanide contraction : The regular decrease in the size of lanthanide ions from La³⁺ to Lu³⁺ is known as lanthanide contraction. It is due to greater effect of the increased nuclear charge than that of the screening effect.

 

Consequences of lanthanide contraction

(a)     It results in slight variation in their chemical properties which helps in their separation by ion exchange

(b)     Each element beyond lanthanum has same atomic radius as that of the element lying above it in the group (e.g. Zr 145 pm, Hf 144 pm); Nb 134 pm, Ta 134 pm ; Mo 129 pm, W 130 pm).

(c)      The covalent character of hydroxides of lanthanides increases as the size decreases from La³⁺ to Lu³⁺. However basic strength decreases. Thus La(OH)₃ is most basic whereas Lu(OH)₃ is least basic. Similarly, the basicity of oxides also decreases in the order from La³⁺ to Lu³⁺.

(d)     Tendency to form stable complexes from La³⁺ to Lu³⁺ increases as the size decreases in that order.

(e)     There is a slight increase in electronegativity of the trivalent ions from La to Lu.

(f)      Since the radius of Yb³⁺ ion (86 pm) is comparable to the heavier lanthanides Tb, Dy, Ho and Er, therefore they occur together in natural minerals.