Question

Consider these two statements regarding lanthanoid contraction and their explanation as understood by me.

Lanthanoid contraction is the greater than expected decrease in the atomic radii of the lanthanides varying from lanthanum to lutetium. This causes the atomic radii of 2nd and transition series to be identical.

My Reason

The reason is attributed to the introduction of the forbital which provides very less shielding to the outer electrons in the sorbital.

Now the second one
Lanthanoid contraction is the gradual and not the abrupt decrease in the atomic and ionic radii of the lanthanides varying from lanthanum to luteitium

My Reason

This cause is attributed to the fact that the size of f
orbitals go on decreasing when we proceed towards right due to increasing nuclear charge. This would increase shielding and hence the drop in radii becomes less abrupt. I implore you to advise on which statement and explantion is correct. Thanks

EDIT The second explanation comes from NCERT CLASS 12 as (It is the text which I use)

The factor responsible for the lanthanoid contraction is somewhat similar to that observed in an ordinary transition series and is attributed to similar cause, i.e., the imperfect shielding of one electron by another in the same set of orbitals. However, the shielding of one 4f electron by another is less than that of one d electron by another, and as the nuclear charge increases along the series, there is fairly regular decrease in the size of the entire 4f orbitals.

The first statement comes from wikipedia here

The lanthanide contraction is the greater-than-expected decrease in atomic radii and ionic radii of the elements in the lanthanide series, from left to right. It is caused by the poor shielding effect of nuclear charge by the 4f electrons along with the expected periodic trend of increasing electronegativity and nuclear charge on moving from left to right

Answer 1

The lanthanoid contraction is a gradual decrease in the atomic and ionic radii across the lanthanides due to poor shielding by 4f electrons, which allows the increasing nuclear charge to pull the outer electrons closer, leading to a decrease in the size of 4f orbitals.

Step-by-step explanation:

The concept of lanthanoid contraction refers to the observed decrease in atomic and ionic radii across the lanthanide series, which is not abrupt but gradual. This phenomenon is due to the imperfect shielding effect of the 4f electrons, which are poor at shielding the outer electrons from the increasing nuclear charge.

The first explanation provided is somewhat simplistic, suggesting a "greater-than-expected decrease" without detailing the gradual nature. The second explanation, aligned with the NCERT Class 12 description, accurately states that the contraction is due to the incremental increase of nuclear charge that overpowers the flawed shielding by the 4f electrons, leading to a regular decrease in ionic radii. This is rooted in the concept of effective nuclear charge (Zeff), where Zeff = Z - shielding.