Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following factors may be regarded as the main cause of lanthanide contraction? \(\quad\) [2005] (a) poor shielding of one of 4 f electrons by another in the subshell (b) effective shielding of one of \(4 \mathrm{f}\) electrons by another in the subshell (c) poor shielding of 5 d-electrons by 4 f electrons (d) greater shielding of 5 d-electrons by 4 f electrons

Step-by-Step Solution

Answer

(a) Poor shielding of one of 4f electrons by another in the subshell.

Step 1: Understanding Lanthanide Contraction

Lanthanide contraction refers to the steady increase in the effective nuclear charge experienced by the electrons, leading to a decrease in atomic and ionic radii of the lanthanide elements as atomic number increases.

Step 2: Exploring 4f Electron Shielding

In the lanthanides, 4f orbitals are being filled. These orbitals have poor shielding effects, meaning they do not effectively shield the nuclear charge from the outer electrons.

Step 3: Analyzing 5d Electron Shielding

Since the shielding by 4f electrons is poor, the outer d and s electrons experience a higher effective nuclear charge which causes the contraction. Hence, poor shielding by 4f electrons is crucial to consider.

Step 4: Choosing the Correct Option

Option (a) "poor shielding of one of 4f electrons by another in the subshell" relates directly to the concept of lanthanide contraction, as it emphasizes the poor shielding effect.

Key Concepts

Effective Nuclear Charge

The effective nuclear charge (often denoted as \(Z_{eff}\)) is essentially the net positive charge experienced by an electron in an atom. It accounts for the actual nuclear charge that pulls on the electrons, minus the shielding effect provided by other electrons. In simple terms, it's how tightly an electron feels the pull from the nucleus, taking into account how other electrons can block that pull.

• The effective nuclear charge influences how strongly electrons are held by the atom, affecting atomic size and reactivity.
• In the lanthanides, the effective nuclear charge increases as more protons are added to the nucleus with each subsequent element. Despite this, because the 4f electrons do not shield the increasing nuclear charge effectively, the result is that the electrons in outer shells feel a more substantial effective nuclear charge.

Thus, the electrons are drawn closer to the nucleus, leading to a decrease in both atomic and ionic radii as we move across the lanthanide series.

Electron Shielding

Electron shielding refers to the phenomenon where inner electrons block outer electrons from feeling the full force of the nucleus's attraction. Shells of electrons in the atom create this shielding effect.

• Inner electrons repel outer electrons, reducing the full nuclear charge's pull.
• In the lanthanide series, 4f electrons have particularly poor shielding efficiency. This is because they are less penetrative and largely localized between the nucleus and outer electrons.

In effective shielding, the inner electron cloud reduces the effective nuclear charge felt by the outermost electrons, often leading to larger atomic radii. However, in the case of lanthanides, the poor shielding by 4f electrons does not offset the increasing nuclear charge, resulting in the contraction of the radii.

Atomic and Ionic Radii

Atomic and ionic radii are measures of the size of an atom or ion. The atomic radius refers to the distance from an atom's nucleus to the outermost electron orbit, while the ionic radius pertains to the size of an ion, which can differ from the atomic radius depending on the gain or loss of electrons.

• Lanthanide contraction is a particular phenomenon observed in the series of lanthanide elements, resulting in smaller atomic and ionic radii compared to expectations.
• As you move through the lanthanide series, the increasing effective nuclear charge pulls the outer electrons closer towards the nucleus, causing this contraction in both atomic and ionic radii.
• This contraction affects the properties and chemistry of lanthanides and adjacent groups in the periodic table.

Such contraction is significant because it makes the later lanthanides appear chemically similar to each other and affects the periodic trends, especially with elements following the lanthanides.