Problem 144 - A Complete Resource Book in Chemistry for JEE Main

Question

Lanthanoid contraction is caused due to (a) the appreciable shielding of outer electrons by \(4 \mathrm{f}\) electrons from the nuclear charge (b) the appreciable shielding on outer electrons by 5d-electrons from the nuclear charge (c) the same effective nuclear charge from Ce to Lu (d) the imperfect shielding of outer electrons by \(4 \mathrm{f}\) electrons from the nuclear charge

Step-by-Step Solution

Answer

(d) the imperfect shielding of outer electrons by 4f electrons from the nuclear charge

Step 1: Understanding Lanthanoid Contraction

Lanthanoid contraction refers to the gradual decrease in the size of lanthanoids (from Ce to Lu) in the periodic table. This occurs despite the fact that additional electrons are being added as we move across this series.

Step 2: Reviewing Electron Shielding

Electron shielding involves the blocking of the effective nuclear charge on outer electrons by the inner electrons. Effective shielding means the outer electrons feel less nuclear charge, whereas imperfect shielding means the outer electrons feel more of the nuclear charge because the inner electrons don't block it effectively.

Step 3: Identifying Contribution of 4f Electrons

In the lanthanoids, the electrons are being added to the 4f sublevel. These 4f electrons do not shield outer electrons effectively because they are more diffused and further from the nucleus, rendering their shielding effect imperfect.

Step 4: Evaluating the Options

Option (a) suggests strong shielding by 4f electrons; option (b) suggests shielding by 5d electrons, which is not the primary reason for contraction; option (c) suggests a constant nuclear charge, which contradicts the observed contraction; option (d) states imperfect shielding by 4f electrons, aligning with the actual cause.

Step 5: Selecting the Correct Option

The correct choice must describe the imperfect shielding effect causing enhanced nuclear attraction on the outer electrons, leading to contraction. Thus, option (d) is correct.

Key Concepts

Electron Shielding

Electron shielding is an important concept in understanding atomic and molecular chemistry. It involves the reduction of the effective nuclear charge on an electron cloud by other electrons that are closer to the nucleus. The "shield" is formed by electrons in inner energy levels.
These inner electrons partially block the attractive force of protons in the nucleus on the electrons in outer shells. As a result, the outer electrons experience a diminished nuclear pull, which is known as the effective nuclear charge.
- **Perfect Electron Shielding:** When inner electrons completely block the nucleus's pull on outer electrons. - **Imperfect Electron Shielding:** When inner electrons do not fully block the nuclear charge, meaning outer electrons feel more of the nucleus's pull.
In lanthanoids, it is this imperfect electron shielding by the 4f electrons that contributes significantly to lanthanoid contraction.

4f Electrons

4f electrons occupy the f-block in the periodic table, particularly in the lanthanide series. These electrons are added to the 4f subshells as we move from cerium (Ce) to lutetium (Lu).
Despite their presence, 4f electrons have a minimal role in shielding outer electrons from the nucleus. This is because of their position further from the core and the relatively diffused shape of their orbitals.
In contrast to s and p electrons, which can effectively shield because of their proximity to the nucleus, the 4f orbitals are not as effective at shielding. This imperfect shielding by 4f electrons means outer electrons experience more of the nuclear charge than they would if effective shielding occurred, resulting in the lanthanoid contraction.
Thus, the presence and behavior of 4f electrons are fundamental to understanding the distinct properties of lanthanoids.

Nuclear Charge

The term 'nuclear charge' refers to the total positive charge of the nucleus, which is equal to the number of protons. This nuclear charge exerts a pull on all electrons in an atom, trying to keep them bound within the electron cloud.
The effective nuclear charge, however, can be altered by electron shielding. In the context of the lanthanoids, each element in the series has a progressively increasing nuclear charge as a proton is added with every step from Ce to Lu.
Yet, due to the imperfect shielding by the 4f electrons, the effective nuclear charge experienced by the outer electrons increases. The added nuclear charge leads to a tightening of the electron cloud, reducing the size of the atoms despite the increasing number of protons and electrons.
Thus, understanding nuclear charge and its interplay with electron shielding is crucial for explaining the lanthanoid contraction.