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 \(\mathrm{Ce}\) to \(\mathrm{Lu}\) (d) the imperfect shielding of outer electrons by \(4 \mathrm{f}\) electrons from the nuclear charge

Step-by-Step Solution

Answer

The cause of lanthanoid contraction is the imperfect shielding by 4f electrons (d).

Step 1: Understanding Lanthanoid Contraction

Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii of the 14 lanthanoid elements (from Ce to Lu). This phenomenon is primarily due to changes in electron shielding and effective nuclear charge.

Step 2: Investigating Shielding Effects

The contraction occurs because the 4f electrons are not very efficient at shielding the outer electrons from the nuclear charge. This leads to a greater effective nuclear charge experienced by the electrons as we move across the lanthanoid series.

Step 3: Analyzing Each Option

We need to evaluate the options given: (a) Suggests appreciable shielding by 4f electrons, which is incorrect as 4f electrons do not shield well. (b) Suggests appreciable shielding by 5d electrons, which is not a primary factor. (c) Suggests the same effective nuclear charge, which is incorrect as there's an increase in effective nuclear charge. (d) Suggests imperfect shielding by 4f electrons, which aligns with the known cause of lanthanoid contraction.

Step 4: Selecting the Correct Answer

Option (d), the imperfect shielding of outer electrons by 4f electrons, correctly describes the cause of lanthanoid contraction, as it results in the increased effective nuclear charge that causes the radii contraction.

Key Concepts

4f electrons

The 4f electrons play a crucial role in understanding the characteristics of lanthanoid elements. These electrons belong to the f-block of the periodic table, where they reside in the 4f subshell. This subshell is buried deep within the atom, making its electrons less effective at screening or protecting the outermost electrons from the positive pull of the nucleus. As a result, the outer electrons feel a stronger attraction toward the nucleus, which is an essential detail in the lanthanoid contraction phenomenon. In the context of lanthanoid contraction, the 4f electrons are relatively poor at shielding due to their diffuse nature and distribution. They do not provide a substantial barrier for the outer electrons, allowing the nuclear charge to affect them significantly. Consequently, as you move from one lanthanoid element to the next, the radius of the atoms decreases slightly, leading to what is known as the lanthanoid contraction.

Shielding effect

The shielding effect, also called the screening effect, is a fundamental concept in chemistry that deals with the reduction of the effective nuclear charge on the electron cloud, due to a difference in attractions exerted by the nucleus on its electrons in different inner shells. Inner electrons block, or shield, the outer electrons from the full effect of the nucleus's charge. In lanthanoid elements, the shielding effect of the 4f electrons is particularly inefficient. This is due to the fact that 4f electrons, despite their number, neither form a very compact electron layer around the nucleus nor effectively cover the outer electrons from the nucleus's electrostatic pull. As a result, the outer electrons experience a relatively higher effective nuclear charge. This contributes to the phenomenon of lanthanoid contraction, where the atomic and ionic sizes decrease across the lanthanoid series.

Effective nuclear charge

The effective nuclear charge is the net positive charge experienced by an electron in an atom. It is influenced by the presence of other electrons, especially those in inner shells that can shield outer electrons from the full positive charge of the nucleus.Mathematically, it is expressed as:\[ Z_{ ext{eff}} = Z - S \]where \( Z \) is the actual charge of the nucleus (or atomic number), and \( S \) is the shielding constant due to inner electrons.In the lanthanoid series, although the number of electrons increases along with the increase in atomic number, the shielding provided by the 4f electrons does not offset the increase in nuclear charge significantly. As a result, the effective nuclear charge increases across the series, pulling the outer electrons closer and leading to smaller atomic radii. This is the underlying cause of the lanthanoid contraction.

Atomic radii

Atomic radii refer to the average distance from the nucleus to the outermost electron cloud of an atom. It is an essential concept in understanding the size and shape of atoms and varies notably across different elements and periods in the periodic table. For lanthanoids, the atomic radii change less dramatically than in other series, predominantly due to poor shielding by the 4f electrons. As the effective nuclear charge increases progressively through the lanthanoid series, the outer electrons are drawn closer to the nucleus. This results in a gradual decrease in both atomic and ionic sizes, known as lanthanoid contraction. Understanding this trend is crucial for predicting chemical behavior and reactivity patterns of elements in the series.

Lanthanoid series

The lanthanoid series comprises 14 elements, from Cerium (Ce) to Lutetium (Lu), located in the f-block of the periodic table. These elements are known for their unique properties and play a vital role in many technological and industrial applications. A characteristic feature of the lanthanoid series is the lanthanoid contraction—a steady reduction in atomic and ionic radii as one moves from Ce to Lu. This contraction is mainly attributed to the ineffective shielding by 4f electrons and the resulting increase in effective nuclear charge. As a result of this contraction, several properties of the lanthanoid elements are affected, such as their ionic bonding, electrode potentials, and the formation of various complexes. Recognizing the implications of lanthanoid contraction is crucial in fields like material science and chemistry, where these elements are widely utilized.