The Lanthanoids: Electronic Configuration, Oxidation state, and Lanthanoid Contraction

The Lanthanoids: How many elements do you think there are in and around us? They can be counted, however counting them on your fingers is tough because there are \(118\) of them. There are some hidden elements (in the periodic table) that cannot be ignored, such as lanthanides. Read the article below to learn more about lanthanoid.

Define Lanthanoids

Lanthanoids are also known as Lanthanoid series, Lanthanides, or Lanthanones. Lanthanoids are elements from a series of elements with increasing atomic numbers, starting with lanthanum or cerium (At. No. \(= 58\)) and ending with lutetium (At. No \(=71\)). The lanthanoid series is also known as the first series of rare-earth metals or the first inner transition series.

This series is characterised by the filling of \(4f\)-orbitals in the atoms and consists of fourteen elements from cerium (\({\rm{Ce,}}\) Atomic number \(= 58\)) to lutetium (\({\rm{Lu,}}\) At. No. \(= 71\)). These elements follow lanthanum (\({\rm{La,}}\) At. No. \(= 57\)) in the periodic table and resemble it strongly in physical and chemical properties. This is why they are called lanthanoids. In the periodic table, two additional rows below the main part of the table as parts of the sixth and seventh rows (periods) of the table are the lanthanoids and actinides.

Electronic Configuration of Lanthanoids

It should be noted that the atoms of the lanthanide elements have an electronic configuration with \(6{{\rm{s}}^{\rm{2}}}\) common, but with variable occupancy of level \(4{\rm{f}},\) but the electronic configurations of all positively travelling ions have the form \(4{{\rm{f}}^{\rm{n}}}\) (\({\rm{n = 1}}\) to \(14\) with the increasing atomic number).

The electronic configuration of Lanthanum and Lanthanoids are as follows:

Oxidation States

The most common oxidation state of lanthanides is \( + 3.\) In addition to this state, there are also lanthanides in the \( + 2\) and \( + 4\) states. In aqueous solutions, the \( + 3\) state is more stable than the \( + 2\) and \( + 4\) states. This is because the sum of the lattice and ionization energies of its triple-positive ions is more negative compared to that of its \( + 2\) and \( + 4\) ions. In the solid-state, too, the combination of ionization and lattice energies is more negative for \( + 3\) state than for the dipositive and tetrapositive states. The tripostive state is, therefore, more common in the solid-state.

Oxidation States of Lanthanoids

Lanthanoid Contraction

The steady decrease in the size of lanthanoid ions \(\left( {{{\rm{M}}^{{\rm{3 + }}}}} \right)\) with the increase in atomic number is called lanthanoid contraction.

Causes of Lanthanoid Contraction

As the atomic number in the Lathanoid series increases, the extra electron fills \({\rm{4f}}\) orbitals for each proton in the nucleus. The \({\rm{4f}}\) electrons ​from inner shells and are quite ineffective at filtering out the nuclear charge. Therefore, there is a gradual increase in the effective nuclear charge. As a result, the attraction of the nucleus to the electrons in the outermost shell increases as the atomic number of the lanthanoids increases, and the electron cloud contracts, which leads to a gradual decrease in the size of the lanthanides as the atomic number increases.

Consequences of Lanthanoid Contraction

Some significant consequences of lanthanoid contraction are given below:

I. Atomic and ionic radii: In general, the atomic and ionic radii in a group increase with increasing atomic number. This fact becomes clear if we compare the values ​​of the elements of the first and second transition series as given below.

Atomic Radii of some Transition Elements

II. Basicity of oxides and hydroxides: Due to the contraction of lanthanides, the basic strength of the oxides and hydroxides of lanthanides decreases with increasing atomic number; it is more acidic and less basic. The accumulated lanthanide contraction further reduces the size of the lanthanide cations. Therefore, as the atomic number increases, the polarization power of the lanthanum cations increases. As a result, the ionic character of the oxides and hydroxides regularly decreases, and they become less and less basic.

III. Density: A reduction in the size of atoms due to lanthanoid contraction increases the densities of all elements placed after lanthanoids abnormally.

IV. Ionisation potential: The phenomenon of lanthanoid contraction also affects the values of the ionisation potential of the elements of the third transition series from tungsten onwards. In the absence of lanthanoid contraction, these values should have been much lower and should have decreased regularly on descending the group.

V. Occurrence of yttrium with heavy lanthanoids: Yttrium is a member of group \(3\) and belongs to the d-block of the periodic table. This element occurs quite unexpectedly in nature together with heavy lanthanoids (Ho, Er, etc.). Lanthanoids belong to the f-block and must appear independently. The unusual event is a consequence of the lanthanum contraction.

Physical and Chemical Properties of Lanthanoids

1. Density: Lanthanoids have high densities in the range of \({\rm{6}}.{\rm{77}}\) to \({\rm{9}}.{\rm{74gc}}{{\rm{m}}^{{\rm{ – 3}}}}.\) Densities generally increase with increasing atomic numbers.

2. Melting and Boiling Points: Lanthanoids have quite high melting points, but there is no clear trend in melting and boiling points from \({\rm{La}}\) to \({\rm{Lu}}.\)

3. Ionisation Enthalpies: Lanthanoids have relatively low enthalpies of ionization. The values ​​of \({{\rm{\Delta }}_{\rm{i}}}{{\rm{H}}_{\rm{I}}}\) and \({{\rm{\Delta }}_{\rm{i}}}{{\rm{H}}_{\rm{II}}}\) are quite comparable with those of the alkaline earth metals, especially calcium. The first ionization enthalpies of lanthanides are \({\rm{500 – 600\, kJ\, mo}}{{\rm{l}}^{{\rm{ – 1}}}}\) and the second about \({\rm{1067 – 1200\, kJ\, mo}}{{\rm{l}}^{{\rm{ – 1}}}}{\rm{.}}\)

4. Electropositive Character: Lanthanoids have a high electropositive character due to low ionization enthalpies.

5. Coloured Ions: Many of the lanthanide ions are coloured both in the solid-state and in solutions. the colour is assigned to the \({\rm{f – f}}\) transitions because they have partially filled f-orbitals. However, the absorption bands are narrow, probably due to excitation within the f-levels.

6. Magnetic Behaviour: The lanthanoid ions other than the \({{\rm{f}}^{\rm{0}}}\) type and the \({{\rm{f}}^{\rm{14}}}\) type are paramagnetic. The paramagnetism arises due to the unpaired electrons in \({\rm{f}}\)-orbitals. The paramagnetism rises to a maximum in neodymium.

7. Atomic/Ionic Radii: The atomic radii and ionic radii of tri-positive lanthanide ions \(\left( {{{\rm{M}}^{{\rm{3 + }}}}} \right)\) show a constant and gradual decrease in movement from \({\rm{La}}\) to \({\rm{Lu}}.\)

8. Chemical reactivity: In terms of their chemical reactivity, the first representatives of the series are as reactive as calcium, but their reactivity decreases with increasing atomic number, and they behave more like aluminium. Many lanthanides react with carbon to form salt carbides and with hydrogen to form salty hydrides. Lanthanoids react with oxygen and sulphur to form oxides \(\left( {{{\rm{M}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}} \right)\) or sulphides \(\left( {{{\rm{M}}_{\rm{2}}}{{\rm{S}}_{\rm{3}}}} \right){\rm{.}}\) Cerium gives \({\rm{Ce}}{{\rm{O}}_{\rm{2}}}{\rm{.}}\) Oxides \(\left( {{{\rm{M}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}} \right)\) react with water to form insoluble hydroxides. Oxides and hydroxides in reaction with \({\rm{C}}{{\rm{O}}_{\rm{2}}}\) produce carbonates. Lanthanide compounds are generally ionic.

9. Complex Formation: Compared to d-block transition elements, lanthanoids form few complexes and less easily. The reluctance of lanthanides towards complexes is due to the following two reasons:
a. The size of the tri-positive lanthanide cations is large compared to that of the transition elements. The large size reduces the electrostatic attraction and reduces the possibility of complex formation.
b. In lanthanide ions, f-orbitals are not available to form hybrid orbitals due to the high energies of orbitals that lead to less covalent bond strength. In the absence of f-orbitals, the possibilities of complex formation are less.

10. Formation of Alloy: Lanthanoids are very dense metals and have high melting points. They easily form alloys with other metals, especially iron. These alloys are very useful because the presence of rare earth elements in them improves the workability of the steel when heated. The two important alloys of rare earth elements are Misch metal and Pyrophoric alloys.

Summary

Lanthanoids are elements from a series of elements with increasing atomic numbers, starting with lanthanum or cerium (At. No. \(=58\)) and ending with lutetium (At. No. \(=71\)). Lanthanide elements have an electronic configuration with common \({\rm{6}}{{\rm{s}}^{\rm{2}}}{\rm{.}}\) The most common oxidation state of lanthanides is \({\rm{ + 3}}.\) The steady decrease in the size of lanthanoid ions \(\left( {{{\rm{M}}^{{\rm{3 + }}}}} \right)\) with the increase in atomic number is called lanthanoid contraction. Many of the lanthanide ions are coloured both in the solid-state and in solutions.

FAQs On the Lanthanoids

Q.1. What is the Lanthanoids?
Ans: Lanthanoids are elements from a series of elements with increasing atomic numbers, starting with lanthanum or cerium (At. No=58) and ending with lutetium (At. No=71).

Q.2. Do lanthanides exist?
Ans: Yes, lanthanoids exist. It exists naturally in many minerals.

Q.3. What are the different oxidation states exhibited by the lanthanoids?
Ans: The most common oxidation state of lanthanides is \({\rm{ + 3}}.\) In addition to this state, there are also lanthanides in the \({\rm{ + 2}}\) and \({\rm{ + 4}}\) states.

Q.4. What is special about lanthanoids?
Ans: Lanthanides are all metals with similar reactivity to elements of group 2.

Q.5. Why are lanthanoids so important?
Ans: Lanthanides are particularly useful in geological analysis because their similar chemical properties but a slightly decreasing ionic radius make them ideal tracers for many geological and environmental processes.

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