What Is Lanthanoid Contraction and Why Does It Occur
Lanthanoid contraction definition - In Chemistry, lanthanoid contraction, also called lanthanide contraction, occurs as the atomic size or the ionic radii of the tripositive lanthanide ions steadily decrease from La to Lu because of the electrons entering the inner (n-2) f orbitals and the increasing nuclear charge. This particular gradual decrease in the size with an increasing atomic number is referred to as lanthanide contraction.
Lanthanide contraction happens to all the 14 elements that are present in the lanthanide series. Cerium(Ce), Praseodymium(Pr), Neodymium(Nd), Promethium(Pm), Samarium(Sm), Europium(Eu), Gadolinium(Gd), Terbium(Tb), Dysprosium(Dy), Holmium(Ho), Erbium(Er), Thulium(Tm), Ytterbium(Yb), and Lutetium(Lu) are the total elements that are included in the series. In accordance with the lanthanide contraction of the mentioned elements, as the atomic number increases, the atomic radius of these elements decreases. This is very easy to compare by taking into consideration Ce and Nd in the periodic table. Ce has the atomic number 58 and Nd has the atomic number 60. Now, look at the graph below to find out the atomic radius of the following elements mentioned above.
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About Lanthanide Contraction
Lanthanide contraction is the steady decrease in the size of the ions and atoms of the rare earth elements with increasing atomic numbers from lanthanum (atomic number 57) to lutetium (with an atomic number 71). For every consecutive atom, the nuclear charge can be more positive by a single unit, accompanied by the corresponding increase in the electron count present in the 4f orbitals surrounding the nucleus.
The 4f electrons imperfectly protect each other from the increased positive charge of the nucleus, resulting in a steady rise in the effective nuclear charge attracting every electron as the lanthanide elements progress, resulting in successive decreases in ionic and atomic radii.
Consequences of Lanthanide Contraction
The following points will depict the effect of lanthanide contraction more clearly.:
Atomic Size
The size of the atom of the third transition series is approximately similar to that of the atom of the second transition series. For example, the radius of Zr = radius of Hf and the radius of Nb = radius of Ta, and so on.
Difficulty in the Separation of Lanthanides
As there is only a small change in the ionic radii of the Lanthanides, their chemical properties are the same. This makes the element's separation in the pure state difficult.
Effect on the Basic Strength of Hydroxides
As the size of the lanthanides decreases from the elements La to Lu, the covalent character of the hydroxides increases, and thus, their basic strength decreases. Therefore, Lu(OH)₃ is said to be least basic, and La (OH)₃ is said to be more basic.
Complex Formation
Due to the smaller size and higher nuclear charge, the tendency to produce coordinate. Complexity increases from the element La³⁺to Lu³⁺.
Electronegativity
It increases from the elements La to Lu.
Ionization Energy
Electron's attraction by the nuclear charge is higher, and thus the Ionization energy of the 5d elements is much larger compared to 4d and 3d. In the 5d series, the total elements except Pt and Au contain a filled s-shell.
Elements from Hafnium to rhenium contain similar Ionization energy, and after that, the Ionization energy increases with the number of shared d-electrons such that Gold and Iridium hold the maximum Ionization Energy.
Case Study
Mercury - the Liquid Metal
At room temperature, mercury is the only metal that remains in its liquid form. The nucleus pulls the 6s valence electrons very close together (due to lanthanide contraction), making the outer s-electrons less involved in metallic bonding.
Formation of Complex
Lanthanides with a 3+ oxidation state have a higher charge to radius ratio and hence a lower charge to radius ratio. As compared to d-block elements, this decreases the ability of lanthanides to form complexes. Still they form complexes with strong chelating agents like EDTA, β-diketones, oxime, and so on. They do not form Pπ-complexes.
Cause of Lanthanide Contraction
The effect of lanthanide contraction results from the poor shielding of nuclear charge (with the attractive nuclear force on electrons) by 4f electrons; the 6s electrons can be drawn towards the nucleus, hence resulting in the smaller atomic radius.
In the case of single-electron atoms, the average separation of an electron from the nucleus is defined by the subshell it belongs to and decreases with an increased charge on the nucleus, where this, in turn, leads to the decrease in atomic radius. Whereas, in the case of multi-electron atoms, the decrease in the radius brought about by an increase in nuclear charge is partially offset by the increasing electrostatic repulsion among the electrons.
A "shielding effect" operates, in which existing electrons shield the outer electrons from the nuclear charge by causing them to undergo less effective charge on the nucleus as more electrons are applied to the outer shells. The inner electrons' shielding effect decreases in the following order: s > p > d > f.
In general, as a specific subshell is filled in a period, the atomic radius decreases. This particular effect is specifically pronounced in the case of lanthanides, as the 4f subshell that is filled across these elements is not more effective at shielding the outer shell (n=5 and n=6) electrons. Therefore, the shielding effect can be less able to counter the decrease in radius caused by an increasing nuclear charge. This leads to the "lanthanide contraction". And, the ionic radius drops from a range of 103 pm for lanthanum (III) to 86.1 pm for lutetium (III).
The relativistic effects have been blamed for up to 10% of the lanthanide contraction.
Shielding Effect on the Atomic Radii
The lanthanide is a result of the poor shielding effect of the 4f electrons. The shielding effect is a phenomenon where the inner shell electrons shield the outer shell electrons from getting attracted by the charge of the nucleus. Therefore, if the shielding effect is not good, then the outer shell electron is attracted by the positively charged nucleus towards itself, and thus, the radii of that atom decrease with the increase in the positive charge or atomic number. Thus, s-shell has a greater shielding effect as compared to the other subshells while f-shell has the least shielding effect. The d and the p shell are in between where the p-shell has more shielding effect than the d-shell.
This can be seen if the elements with the f-subshell are compared with the d-block elements that do not have any f-subshell in their atom. Such elements are Pd and Pt. The Pd element has 4d electrons whereas Pt has 5d and 4f electrons. These two elements, therefore, have roughly had the same electronic radii. This is due to the shielding effect and the lanthanide contraction. It is because we expect the Pt to have a larger radius as compared to Pd since it has more number of electrons and larger number of protons but not because of the poor shielding effect of the f-subshell present. Hence, this will increase the nucleus pull of the electrons towards the nucleus.
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FAQs on Lanthanoid Contraction in the Periodic Table
1. What is lanthanoid contraction?
The lanthanoid contraction is the gradual decrease in the atomic and ionic radii of lanthanoids from La (Z = 57) to Lu (Z = 71) with increasing atomic number.
- It occurs across the 4f series of the periodic table.
- Despite increasing nuclear charge, electrons are added to the 4f orbitals.
- The poor shielding by 4f electrons causes a stronger attraction between the nucleus and outer electrons.
- This results in a steady reduction in size across the lanthanoid series.
2. Why does lanthanoid contraction occur?
Lanthanoid contraction occurs because 4f electrons shield the nuclear charge very poorly, leading to an increased effective nuclear charge across the series.
- As atomic number increases, protons are added to the nucleus.
- Electrons enter the 4f subshell.
- 4f orbitals have poor shielding ability compared to s and p orbitals.
- The outer electrons experience greater nuclear attraction, reducing atomic and ionic radii.
3. What is the effect of lanthanoid contraction on atomic and ionic radii?
The main effect of lanthanoid contraction is a steady decrease in both atomic radii and ionic radii of lanthanoids across the series.
- The ionic radius of La3+ is larger than that of Lu3+.
- The decrease is gradual and regular.
- The contraction influences bonding, density, and complex formation.
4. How does lanthanoid contraction affect the properties of elements?
Lanthanoid contraction significantly affects the physical and chemical properties of elements, especially size, density, and reactivity.
- Atomic size decreases gradually across the series.
- Density increases due to decreasing size and increasing atomic mass.
- Chemical properties of lanthanoids become very similar.
- It influences the size of elements following lanthanoids, such as hafnium (Hf).
5. What is the impact of lanthanoid contraction on transition elements?
The lanthanoid contraction causes elements of the second and third transition series to have nearly identical atomic radii.
- For example, Zr (Z = 40) and Hf (Z = 72) have almost the same size.
- This similarity results from contraction before the 5d series begins.
- As a result, these elements show very similar chemical properties.
6. Why are zirconium and hafnium similar due to lanthanoid contraction?
Zirconium and hafnium are similar because lanthanoid contraction reduces the atomic radius of hafnium to nearly that of zirconium.
- Zr belongs to the 4d series, while Hf belongs to the 5d series.
- Normally, 5d elements should be larger than 4d elements.
- However, contraction in the lanthanoid series before Hf reduces its size.
- Both therefore have nearly identical radii and similar chemical behavior.
7. Does lanthanoid contraction affect basic strength of hydroxides?
Yes, lanthanoid contraction decreases the basic strength of lanthanoid hydroxides from La(OH)3 to Lu(OH)3.
- As ionic radius decreases, charge density increases.
- Higher charge density strengthens attraction between Ln3+ and OH-.
- This reduces the tendency to release OH- ions.
- Thus, basicity decreases across the series.
8. How does lanthanoid contraction influence complex formation?
Lanthanoid contraction increases the tendency of lanthanoids to form complexes because smaller ions have higher charge density.
- From La3+ to Lu3+, ionic radius decreases.
- Charge density increases as size decreases.
- Higher charge density enhances attraction toward ligands.
- Therefore, later lanthanoids form more stable coordination complexes.
9. What is the electronic configuration responsible for lanthanoid contraction?
Lanthanoid contraction is associated with the gradual filling of the 4f orbitals in the general electronic configuration [Xe] 4f1–14 5d0–1 6s2.
- Electrons are added to the inner 4f subshell.
- 4f electrons have poor shielding effect.
- Effective nuclear charge increases across the series.
- This causes contraction in atomic and ionic radii.
10. What are the important consequences of lanthanoid contraction?
The important consequences of lanthanoid contraction include similar sizes of 4d and 5d elements, decreased basicity, and increased complex formation tendency.
- Similarity between Zr and Hf.
- Decrease in ionic radii from La3+ to Lu3+.
- Gradual decrease in basic strength of Ln(OH)3.
- Increased stability of coordination complexes.
