Question

The increasing order of the following elements (atomic numbers given) with respect to atomic radius is \[ Eu(63),\quad Ho(67),\quad La(57),\quad Yb(70) \]

• A. \(La \lt Eu \lt Ho \lt Yb\)
• B. \(Yb \lt Ho \lt Eu \lt La\)
• C. \(Yb \lt Ho \lt La \lt Eu\)
• D. \(Ho \lt Yb \lt La \lt Eu\)

Hint:

Lanthanoid contraction: \[ La \gt Ce \gt Pr \gt \cdots \gt Lu \] Atomic radius decreases steadily across the lanthanoid series due to poor shielding by \(4f\)-electrons.

Answer 1

The Correct Option is B

Concept: Lanthanoids exhibit a gradual decrease in atomic radius with increasing atomic number. This phenomenon is known as

lanthanoid contraction. It occurs because the \(4f\) electrons shield the nuclear charge very poorly, resulting in a stronger attraction between the nucleus and outer electrons.

Step 1: Apply the concept of lanthanoid contraction. As atomic number increases, \[\begin{aligned} \text{Atomic radius decreases} \end{aligned}\] Among the given elements: \[\begin{aligned} La(57) &\gt Eu(63) \gt Ho(67) \gt Yb(70) \end{aligned}\]

Step 2: Arrange in increasing order of atomic radius. The smallest radius corresponds to the highest atomic number. \[\begin{aligned} Yb \lt Ho \lt Eu \lt La \end{aligned}\] \[\begin{aligned} \boxed{Yb \lt Ho \lt Eu \lt La} \end{aligned}\] Hence, option \(\mathbf{(B)}\) is correct.