Lanthanides - Rare Earth Metals

The Lanthanides are rare earth elements of the modern periodic table. It means that the elements with atomic numbers ranging from 58 to 71 follow the element Lanthanum. They are known as the rare earth metals since the occurrence of these elements is minimal (3 × 10-4 % of the Earth’s crust). They are found in ‘monazite’ sand’ as the lanthanide orthophosphates.

The word ‘lanthanide’ was first introduced in 1925 by the Norwegian mineralogist Victor Goldschmidt. The lanthanide family contains fifteen metallic elements (starting from lanthanum to lutetium), all but one of which are the f-block elements. The valence electrons of these elements also lie in the 4f orbital. However, lanthanum is a d-block element having an electronic configuration of [Xe]5d16s2.

The lanthanides are high dense elements, with densities ranging roughly from 6.1 to 9.8 grams per cubic centimetre. Like most metals, these elements have high melting points (roughly from 800 to 1600 degrees Celsius) and high boiling points (roughly from 1200 to 3500° Celsius). All the lanthanides are well known to form Ln3+ cations.

Lanthanides are also highly dense metals even with higher melting points than d-block elements. They produce alloys with other metals. These are the f block elements, which are also referred to as the inner transition metals. The inner transition ions/elements may have the electrons in s, d, and f- orbitals.

Lanthanides Contraction

The ionic radii or the atomic size of tripositive lanthanide ions steadily decrease from La to Lu due to the increasing nuclear charge and electrons entering the inner (n-2) f orbital. This gradual size decrease with an increasing atomic number is known as lanthanides contraction.

Consequences of Lanthanides Contraction

The points listed below will depict the effect of lanthanide contraction.

• Atomic Size - The third size transition atom series is nearly the same as the atom of the second transition series. For example, the radius of Nb = radius of Ta and a radius of Zr = radius of Hf, etc.

• Difficulty in the Separation of Lanthanides - As there is only a small change in the ionic radii of the Lanthanides and their chemical properties are same, this makes the separation of lanthanides elements difficult in the pure state.

• Effect on the Basic Strength of Hydroxides - As the lanthanides’ size decreases from La to Lu and the hydroxides’ covalent character increases, hence their basic strength decreases. Therefore, Lu(OH)3 is the least basic and La (OH)3 is more basic.

• Complex Formation - Due to the smaller size and higher nuclear charge, they tend to form coordinates. Complexes raise from La3+ to Lu3+.

Properties of Lanthanide Series

Like transition metal In the periodic table, if we consider lanthanides and actinides series, the table will be too wide. These two series are called 4f or Lanthanide series and 5f or Actinides series which are present at the bottom of the periodic table. Together, the 4f and 5f series are called inner transition elements.

All elements in the series closely resemble lanthanum and each other in their physical and chemical properties. Some key characteristics of lanthanides are listed below.

• They are silvery in appearance and have a lustre.

• They are soft metals and can be even cut with a knife.

• The lanthanide elements have different reaction tendencies on basicity dependence. Some take time to react while some are very reactive.

• Lanthanides can become brittle or corrode if they are contaminated with other metals or non-metals.

• They are magnetic.

• Mostly, they form a trivalent compound and sometimes they can form divalent or tetravalent compounds as well.

Physical Properties of Lanthanides

The physical properties of Lanthanides are given in the following.

• Melting and Boiling Points - These have a high melting point relatively, but there is no definite trend in the boiling and melting point of lanthanides.

• Density - Being the ratio of the mass of the substance to its volume, d-block elements’ density will be more than the s-block elements. The density trend will be the reverse of atomic radii. Among the inner transition series, the density increases with an increasing atomic number along the period.

They have a high density between 6.77 to 9.74 g cm-3. It increases with the increasing atomic number.

Magnetic Properties - Materials are classified based on their interaction with the magnetic field as Diamagnetic (if repelled), Paramagnetic (if attracted).

The lanthanide ions/atoms other than f0 and f14 type are paramagnetic in nature because of the unpaired electrons in orbitals. Therefore, Yb2+, Ce4+, and Lu3+ are diamagnetic.

Unpaired electrons contribute to ‘spin magnetic moment’ and ‘orbital magnetic moment.’ Both moments of the electrons are taken into account for calculating the total magnetic moment.

\[Μ = \sqrt{[4S(S+1)+L(L+1)]} BM\]

Its unit is BM (Bohr Magneton).

Electronic Configuration of Lanthanides

The first f-block Lanthanides contains a terminal electronic configuration of [Xe] 4f1-14 5d 0 - 16s2 of the 14 lanthanides. Promethium (Pm) having an atomic number of 61, is the only synthetic radioactive element. The energy of 5d and 4f electrons are almost close to each other, so the 5d orbital lives vacant and the electrons enter into the 4f orbital.

Exceptions are in the gadolinium case i.e. Gd (Z = 64), where the electron enters the 5d orbital because of the presence of lutetium (Z = 71) and half-filled d-orbital enters the 5d orbital.