Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The effect of lanthanoid contraction in the lanthanoid series of elements by and large means: (a) increase in both atomic and ionic radii (b) decrease in atomic radii and increase in ionic radii (c) decrease in both atomic and ionic radii (d) increase in atomic radii and decrease in ionic radii

Step-by-Step Solution

Answer

(c) decrease in both atomic and ionic radii

Step 1: Understanding Lanthanoid Contraction

Lanthanoid contraction refers to the phenomenon where there is a gradual decrease in the size of the atoms and ions of the lanthanide elements (from Lanthanum, La, to Lutetium, Lu) as the atomic number increases.

Step 2: Effect on Atomic Radii

As we move across the lanthanide series, each successive element has an additional proton and electron. The additional electron enters an f-orbital, which is poorly shielding the increased nuclear charge. This results in a strong attraction between the electrons and the nucleus, causing a decrease in atomic radii.

Step 3: Effect on Ionic Radii

Similarly, this poorly-shielded nuclear attraction causes a decrease in ionic radii as well, because the ions also fail to shield effectively. Thus, both the atomic and ionic radii decrease across the series.

Step 4: Conclusion based on Analysis

Considering Steps 2 and 3, the lanthanoid contraction results in a decrease in both atomic and ionic radii across the series (La to Lu).

Key Concepts

Atomic Radii

Understanding atomic radii is crucial when studying elements. Atomic radius is the distance from the center of an atom's nucleus to the outermost shell of electrons. It provides insight into the size of an atom. In the lanthanoide series, there is an intriguing phenomenon called lanthanoid contraction. This results in the reduction of atomic radii across the series, from Lanthanum (La) to Lutetium (Lu).

• With each step across this series, an additional proton is added to the nucleus.
• This also means adding an electron, which enters into the f-orbitals.

The electrons in these f-orbitals are not very effective in shielding the added nuclear charge. Therefore, the increased positive charge pulls the existing electrons closer to the nucleus, thus reducing the atomic radii.

Ionic Radii

Ionic radii refer to the size of an ion, which can be different from that of the corresponding neutral atom due to loss or gain of electrons.

In the lanthanoid series, as with atoms, the ions also experience shrinkage in their size. This decrease in ionic radii is influenced similarly by the ineffective shielding of electrons in the f-orbitals.

• When an atom loses electrons to form a cation, it reduces its radius. However, the poor shielding effect of the f-electrons allows the nucleus's increased positive charge to exert a greater pull on the remaining electrons.
• This stronger pull further compresses the ionic structure, leading to smaller ionic radii across the sequence.

Thus, both atomic and ionic radii show a trend of decreasing size from Lanthanum through to Lutetium.

f-orbitals

The f-orbitals are one of the types of orbitals that electrons can occupy in an atom. These orbitals are more complex than s, p, and d orbitals, and they have a unique shape that affects their properties, like shielding.

• In the case of lanthanides, electrons progressively fill the f-orbitals.
• These f-orbitals have poor shielding capabilities, meaning they do not effectively block the increasing positive charge from the nucleus as electrons are added with each subsequent element in the series.

This ineffective shielding leads to the lanthanoid contraction, causing the observed decrease in atomic and ionic radii. Understanding the role of f-orbitals is vital for explaining why atoms and ions in this series become smaller despite having additional protons and electrons.