Question

Atomic radii and ionic radius of lanthanoid series increases from La to Lu due to?

• A. Lanthanoid contraction
• B. Actinoid contraction
• C. Intermolecular bonding
• D. None

Hint:

Lanthanoid contraction arises because \(4f\)-electrons shield the nuclear charge very poorly. This causes a steady decrease in atomic and ionic sizes across the lanthanoid series.

Answer 1

The Correct Option is A

Concept: The lanthanoids comprise fourteen elements from cerium (\(Z=58\)) to lutetium (\(Z=71\)). As we move across the lanthanoid series, electrons are progressively added to the \(4f\)-subshell. The \(4f\)-electrons are poor at shielding the nuclear charge. Consequently, the effective nuclear charge experienced by the outer electrons increases steadily from left to right in the series. As a result, the electrons are pulled closer towards the nucleus, causing a gradual decrease in both atomic and ionic radii. This gradual decrease in size across the lanthanoid series is known as lanthanoid contraction.

Step 1: Understand the filling of the \(4f\)-orbitals. As we move from La to Lu, electrons enter the \(4f\)-orbitals. \[ La \rightarrow Ce \rightarrow \cdots \rightarrow Lu \] The \(4f\)-electrons do not effectively shield the increasing nuclear charge.

Step 2: Examine the effect on atomic size. Due to the increase in effective nuclear charge, the outer electrons experience stronger attraction toward the nucleus. Hence, \[ \text{Atomic radius decreases} \] and \[ \text{Ionic radius decreases} \] throughout the series.

Step 3: Identify the phenomenon responsible. The observed decrease in atomic and ionic radii from La to Lu is called: \[ \boxed{\text{Lanthanoid Contraction}} \] Therefore, the correct option is \[ \boxed{(1)} \]