Lanthanides - Rare Earth Metals

What are Lanthanides?

The rare earth elements of the modern periodic table are known as lanthanides i.e. the elements with atomic numbers from 58 to 71 following element Lanthanum. Since the occurrence (3×10-4 % of earth’s crust) of these elements are very small they are called also like the rare earth metals. They are available in ‘monazite’ sand’ as lanthanide orthophosphates.

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

Properties of Lanthanide Series

All of the elements in the series closely resemble lanthanum and each another in their chemical and physical properties. Some of the key characteristics and properties are:

  • They have a lustre and are silvery in appearance.
  • They are soft metals and can even be cut with a knife
  • The elements have different reaction tendencies depending on basicity. Some are very reactive while some take time to react.
  • Lanthanides can corrode or become brittle if they are contaminated with other metals or non-metals.
  • They all mostly form a trivalent compound. Sometimes they can also form divalent or tetravalent compounds.
  • They are magnetic.

Lanthanide Contraction

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

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Consequences of Lanthanide Contraction

Following points will clearly depict the effect of lanthanide contraction:

  • Atomic size
  • Difficulty in the separation of lanthanides
  • Effect on the basic strength of hydroxides
  • Complex formation
  • The ionization energy of d-block elements

1. Atomic size: Size of the atom of third transition series is nearly the same as that of the atom of the second transition series. For example: radius of Zr = radius of Hf & radius of Nb = radius of Ta etc.

2. Difficulty in the separation of lanthanides: As there is an only small change in the ionic radii of Lanthanides, their chemical properties are similar. This makes the separation of elements in the pure state difficult.

3. Effect on the basic strength of hydroxides: As the size of lanthanides decreases from La to Lu, the covalent character of the hydroxides increases and hence their basic strength decreases. Thus, La (OH)3 is more basic and Lu(OH)3 is the least basic.

4. Complex formation: Because of the smaller size but higher nuclear charge, tendency to form coordinate. Complexes increases from La3+ to Lu3+.

5. Electronegativity: It increases from La to Lu.

6. Ionization energy: Attraction of electrons by the nuclear charge is much higher and hence Ionization energy of 5d elements are much larger than 4d and 3d. In 5d series, all elements except Pt and Au have filled s-shell.

Elements from Hafnium to rhenium have same IE and after IE increases with the number of shared d-electrons such that Iridium and Gold have the maximum IE.

Case Study:

Mercury – the liquid metal: Mercury is the only metal that exists in its liquid state at room temperature. 6s valence electrons of Mercury are more closely pulled by the nucleus (lanthanide contraction) such that outer s-electrons are less involved in metallic bonding.

7. Formation of Complex: Lanthanides exhibiting 3+ oxidation state is the larger and hence low charge to radius ratio. This reduces the complex forming ability of lanthanides compared to d-block elements. Still they, form complexes with strong chelating agents like EDTA, β-diketones, oxime etc. They do not form Pπ-complexes.

Electronic Configuration of Lanthanides

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

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

Oxidation State of Lanthanides

All the elements in the lanthanide series show an oxidation state of +3. Earlier it was believed that some of the metals (samarium, europium, and ytterbium) also show +2 oxidation states. Further studies on these metals and their compounds have revealed that all the metals in lanthanide series exhibit +2 oxidation state in their complexes in solutions.

A few metals in the lanthanide series occasionally show +4 oxidation states. This uneven distribution of oxidation state among the metals is attributed to the high stability of empty, half-filled or fully filled f-subshells.

The stability of f-subshell affects the oxidation state of lanthanides in such a way that the +4 oxidation state of cerium is favoured as it acquires a noble gas configuration but it reverts to a +3 oxidation state and thus acts as a strong oxidant and can even oxidize water, although the reaction will be slow.

The +4 oxidation state is also exhibited by the oxides of:

Europium (atomic number 63) has the electronic configuration [Xe] 4f7 6s2, it loses two electrons from 6s energy level and attains the highly stable, half-filled 4f7 configuration and hence it readily forms Eu2+ion. Eu2+ then changes to the common oxidation states of lanthanides (+3) and forms Eu3+, acting as a strong reducing agent.

Ytterbium (atomic number 70) also has similar reasons for being a strong reducing agent, in the Yb2+ state; it has a fully filled f-orbital.

The presence of f-subshell has a great influence on the oxidation state exhibited by these metals and their properties. New developments and findings continue to add information on lanthanides.

The energy gap between 4f and 5d orbitals is large and so the number of oxidation states limited, unlike the d-block elements.

Why Lanthanide show Variable Oxidation State?

Lanthanides show variable oxidation states. They also show +2, +3, and +4 oxidation states. But the most stable oxidation state of Lanthanides is +3. Elements in other states hence try to lose or gain electrons to get +3 state. By that those ions become strong reducing or oxidizing agents respectively.

Oxidation state in Aqueous Solution

In aqueous solution, Sm2+, Eu2+ and Yb2+ loose electron, ie get oxidized and are good reducing agents. On the other hand Ce4+, Pr4+, Tb4+ gain electron – gets reduced and are good oxidizing agents. Higher oxidation states (+4) of elements are possible only with oxides. Example: Pr, Nd, Tb and Dy.

Chemical Reactivity of Lanthanides

All the lanthanides show similarity in the reactivity but are greater than the transition elements. This is due to the shielding of unpaired electrons of the inner 4f-orbital by the outer 5s, 5p, and 5d orbital’s.

Get readily tarnished with oxygen and forms the oxides of M2O3 except for CeO2 which reacts with hydrogen forming solid hydrides at 300-400 C.

Hydrides get decomposed by water. Halides can be made by heating metal with halogen or the oxide with ammonium halide. Chlorides are deliquescent while fluorides are insoluble. Nitrates, acetates, sulphates are soluble while carbonate, phosphate, chromates and oxalates are insoluble in water.

Ionization Energy of Lanthanides

Ionization energy is the energy needed to remove the valence electron from the atom/ion and is directly related to the force of attraction on the electron. Hence larger the nuclear charge and smaller the radii of the electron larger will be the ionization energy (IE). Also, the ionization energy will be more for half-filled and fully filled orbitals.

IE of the lanthanides elements is larger than s-block and smaller than the d block elements, between which, they are placed.

Physical Properties of Lanthanides

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

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

2. Melting and Boiling Points: They have a fairly high melting point but there is no definite trend in the melting and boiling point of lanthanides.

3. Magnetic Properties: Materials are classified by their interaction with the magnetic field as:

  • Diamagnetic if repelled
  • Paramagnetic if attracted

The lanthanide atoms/ions other than f0 and f14 type are paramagnetic in nature due to unpaired electrons in orbitals. Hence Lu3+, Yb2+ and Ce4+ are diamagnetic.

Unpaired electrons contribute to ‘orbital magnetic moment’ and ‘spin magnetic moment’. Orbital angular moment and spin magnetic moment of the electrons are taken into account for calculating the total magnetic moment.
Μ = √[4S(S+1)+L(L+1)] BM and its unit is Bohr Magneton (BM)

Formation of Coloured Ions

Lanthanides ions can have electrons in f-orbital and also empty orbitals like the d-block elements. When a frequency of light is absorbed, the light transmitted exhibit a colour complementary to the frequency absorbed. Inner transition element ions can absorb the frequency in the visible region to use it for f-f electron transition and produce visible colour.

Many of the lanthanide metals are silver white. The lanthanide ions with +3 oxidation state are coloured both in solid state and in aqueous solution.

The colour of a cation depends on the number of unpaired f electrons Lanthanides, with xf electrons, have the same colour as of (14-x) electron elements.

Uses of Lanthanides

  • Metallurgical applications: Some of the alloys of lanthanide elements find important metallurgical applications as reducing agents. Example: Misch metals (Ce- 30 to 35%)
  • Ceramic applications: Ce(III) and Ce(IV) oxides find use in glass polishing powders whereas Nd and Pr oxides are extensively used in colouring glass and in the production of standard light filters.
  • Catalytic applications: Some lanthanide compounds are used as catalysts. Example: Cerium phosphate is used in petroleum cracking as a catalyst.
  • Electronic applications: The ferromagnetic garnets of 3Ln2O3.5Fe2O3 type are used in microwave devices.
  • Nuclear applications: These elements and some of their compounds are used in nuclear control devices, shielding devices and fluxing devices. Sm – 140, Eu – 153, Gd- 155, Gd- 157 and Dy- 164 are some of the important isotopes used in nuclear technology.
  • Oxides are useful as phosphors in fluorescent materials.
  • Ceramic sulphate is a good analytical oxidizing agent.

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