Lanthanide Contraction-Definition, Causes, Consequences, Practice Problems, FAQ

Do you know what is ‘trend’ is? Everyone has been trying to use Artificial Intelligence or Machine Learning to create quantitative predictions for the past 5 years, to know the trend of the stock prices.

But in case it doesn’t happen, what can you say? It goes off-trend, right? There are trends in every business, and innovation is what propels the train to modify products, processes, services, and overall plans. If a company does not adapt its practices to be more innovative, it will be left behind in the market.

Chemistry also covers some trends but there are a few important concepts which go off trend. Lanthanide contraction is one of the similar cases which we will study here in detail. So, let’s begin!

Table of content:

• What are lanthanides?
• What is lanthanide contraction?
• Causes of lanthanide contraction.
• Consequences of lanthanide contraction.
• Practice Problems
• Frequently asked questions-FAQ

What are lanthanides?

Because of their striking resemblance to the element lanthanum (atomic no: 57), the f-block (inner transition) elements with partially filled 4f-subshells are known as Lanthanides or Lanthanones Lanthanides are a group of fourteen elements with atomic numbers ranging from 58 to 71.

What is lanthanide contraction?

• The Lanthanide Contraction is a term used to characterise the atomic radius trend seen in the Lanthanide series. 
• Another essential aspect of the Lanthanide Contraction is that the 5s and 5p orbitals penetrate the 4f sub-shell, allowing the 4f orbital to be exposed to the rising nuclear change, causing the atomic radius to shrink. Throughout the series, the size of the characters decreases.
• Due to the geometry of f-orbitals, the shielding of one f- electron by another from the influence of the nuclear charge is extremely minimal, and so with rising atomic number, the effective nuclear charge experienced by every 4f electron increases, as atomic or ionic radii contracts from Lanthanum (La) to Lutetium (Lu). 
• Lanthanide Contraction is the decrease of atomic or ionic radii. 
• Lanthanides have chemical characteristics that are nearly identical due to Lanthanide contraction.

Causes of lanthanide contraction

• In lanthanides, additional electrons enter the 4f-sub shell of Lanthanides, but not the valence shell, which is the sixth shell.
• Because the geometry of the f sub-shell is very diffused, the shielding effect of one electron in the 4f - sub shell by another in the same sub-shell (i.e. mutual shielding effect of 4f- electrons) is even smaller than that of d-electrons. Each step increases the nuclear charge (atomic number) by one.
• As a result, the nuclear charge grows with each step, but the mutual shielding effect of the 4f - electron does not. As a result of the enlarging nucleus, electrons in the outermost shell feel increasing nuclear attraction. As a result, as we progress from La (57) to Lu (71), the atomic and ionic radii continue to decrease.
• As a result of the lanthanoid contraction, the second and third d series have very similar radii (e.g., Zr (160 pm), Hf (159 pm), physical and chemical properties, which is considerably more than one would predict based on the general periodic relationship.

The consequence of lanthanide contraction

1. Shielding and its effect on atomic radius:

• The Lanthanide Contraction is caused by the 4f electrons' weak shielding effect. 
• The shielding effect is a phenomenon in which inner-shell electrons buffer outer-shell electrons from nuclear charge. 
• When the shielding is poor, the positively charged nucleus attracts the electrons more strongly, reducing the atomic radius as the atomic number rises. The s orbital has the most shielding and the f orbital has the least, and the p and d orbitals are in the middle, with p being greater than d. The order of shielding decreases as 

s > p > d > f

• For example, the atomic radius of Zr (160 pm), Hf (159 pm) is almost similar due to lanthanide contraction.

2. Effect of ionization energies and its properties:

• The ionisation energy increases as the number of protons increases and the atomic radius falls. 
• This is owing to a more positively charged nucleus and a stronger nuclear pull on electrons. An increased effective nuclear charge causes a stronger pull. The nucleus has a larger positive charge than the electron's negative charge, resulting in effective nuclear charge (net positive charge). 
• The melting point, and hardness of the Lanthanide Series increase from left to right.

3. Density of post lanthanide elements are high:

• The post-lanthanide elements' atomic sizes shrink as a result of lanthanide contraction. The atoms in a metallic lattice are packed so tightly together that the densities are extremely high.
• The density of the second transition series is slightly greater than the density of the first transition series, whereas the density of the third transition series is nearly double that of the second transition series.

4. Basic character of oxides and hydroxides:

• With increasing atomic number, the basic strength of lanthanide oxides and hydroxides decreases. As ionic radii shrink, basicity diminishes. The basicity of Ln⁺³ ions should decreases in the following order: La⁺³ > Ce⁺³ > Pr⁺³ ....> Lu⁺³.
• We can see the difference in basicity and is reflected in following cases.
• The thermal breakdown of oxy-salts reflects these changes in basicity. More basic oxy- salts less quickly it gets breakdown.
• Ion hydrolysis- more basic ions hydrolyze less readily.
• salt solubilities.
• complex formation
• Decreasing ease of oxidation of metals with rising atomic number — oxidation potential for the pair Ln --> Ln⁺³ +3e^- regularly decreases.
• The decrease in size of Ln⁺³ ions from La⁺³ to Lu⁺³ because of lanthanide contraction,it enhances the covalent character (i.e. decreases the ionic character) between Ln⁺³ and OH^- ions in Ln(III) hydroxides.
• Similarly, when the atomic number of the Ln -atom increases, the basic strength of the oxides, Ln₂O₃, decreases.
• Lanthanides can be separated via fractional crystallisation and basicity changes due to small variations in characteristics caused by Lanthanide contraction.
• The elements Zr-Hf and Nb-Ta are almost identical in size and characteristics properties than that of the first and second transition series, for example, their salt solubilities are quite similar.
• Occurrence of Y with heavy lanthanides: Y⁺³ and Er⁺³ have the same crystal radii (Y⁺³ = 0.93 Å and Er⁺³ = 0.96 Å ). The invariable presence of Y with heavier Lanthanides is due to the similarities in atomic size of these two cations, as well as the equality in ionic charge (= +3 in both ions).

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Practice Problems:

Q1. The general electronic configuration of Lanthanides can be written as

A. [Rn]4f^1-145d^0-26s²
B. [Rn]4f^1-145d^0-16s²
C. [Xe]4f^1-145d^0-16s²
D. [Xe]4f^1-145d^0-26s²

Answer: C
Solution: The general electronic configuration of lanthanides can be written as [Xe]4f^1-145d^0-16s²

Q2. Most of the lanthanides exhibit in +3 oxidation state because ____________

A. it is easy to remove 3 electrons from 4f subshell
B. it is easy to remove 3 electrons from 5f subshell
C. it is easy to remove 2 electrons from 6s subshell and 1 electron from 4f subshell
D. it is easy to remove 2 electrons from 5d subshell and 1 electron from 4f subshell

Answer: C
Solution: Most of the Lanthanides exhibit in +3 oxidation state because it is easy to remove 2 electrons from 6s and one electron from 4f subshell.

Q3. _________________ acts as a strong reducing agent .

A. Europium
B. Cerium
C. Terbium
D. Lanthanum

Answer: A
Solution: Europium (atomic number 63) has the electronic configuration [Xe]4f⁷6s², in which it loses two electrons from the 6s energy level and achieves the very stable, half-filled 4f⁷ configuration, from which it quickly forms the Eu⁺² ion. The typical oxidation states of lanthanides (+3) are then converted to Eu⁺³, which acts as a powerful reducing agent.

Q4. What gives lanthanides their distinctive colour?

Solution: The presence of partially filled f orbitals gives lanthanide ions their colour. As a result, specific wavelengths in the visible part of the spectrum can be absorbed. This causes f-f transitions, which are transitions from one 4f orbital to another 4f orbital of the same atom/ion.

Frequently asked questions-FAQ

Question 1. Are lanthanides good conductors of heat or electricity?
Solution: lanthanide good conductor of heat or electricity because the resistivity of lanthanides is relatively high.

Question 2. Can we use lanthanides for making alloys?
Solution: Lanthanides have long been used as alloys to provide metals strength and hardness. Cerium is the most common lanthanide utilised in this application, along with tiny amounts of lanthanum, neodymium, and praseodymium.

Question 3. What is the other name of lanthanides?
Solution: Lanthanides are also called rare earth metals. They were given this name because they are widely distributed around the globe, making it difficult to find a large number in one location. Because promethium is radioactive and decays, it is extremely rare.

Question 4. Mention some lanthanide elements which are beneficially for health?
Solution: Er and Ce salts increase the number of red blood cells and the amount of haemoglobin in the blood, hence they are beneficial for health.

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