The Lanthanoids

The question pertains to the lanthanoid contraction, an important concept in the periodic trends of the lanthanide series of elements. Let’s break down the concept to understand the correct answer.

Explanation:

1. Definition of Lanthanoid Contraction: It refers to the gradual reduction in the atomic and ionic radii of the lanthanide series elements, from lanthanum (La) to lutetium (Lu), as the atomic number increases.

2. Cause: This contraction is primarily due to the poor shielding effect of the 4f electrons. The 4f electrons do not shield the nuclear charge effectively, resulting in a greater effective nuclear charge. This causes the electrons to be pulled closer to the nucleus.

3. Effect on Atomic and Ionic Radii: Because of this increased effective nuclear charge:

   • Both atomic and ionic radii decrease as we move across the lanthanide series.

4. Conclusion: The correct option that aligns with this concept is "decrease in both atomic and ionic radii".

Justification:

Let's examine the given options:

• Decrease in both atomic and ionic radii: This is the correct explanation for the lanthanoid contraction as discussed.
• Increase in atomic radii and decrease in ionic radii: This is incorrect as both radii decrease.
• Increase in both atomic and ionic radii: Incorrect; neither radii increase due to the effective nuclear charge.
• Decrease in atomic radii and increase in ionic radii: Incorrect as both radii decrease.

Conclusion:

The effect of lanthanoid contraction is the decrease in both atomic and ionic radii. This is due to inefficient shielding by 4f electrons, thereby causing a stronger pull by the nucleus on the valence electrons.