Queries regarding Lanthanoid Contraction

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 $f$ orbital which provides very less shielding to the outer electrons in the $s$ orbital.

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

First, atomic radius is not as well defined as one might hope (from wikipedia):

The radius of an atom is not a uniquely defined property and depends on the definition. Data derived from other sources with different assumptions cannot be compared

If we plot the theoretical atomic radii by period, we can see that there is a general trend of a decrease of the radius within a period (source of data: https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)):

(The value shown in the x-axis is the column number in the long form periodic table, i.e. the number of protons added since the "last" noble gas element.)

The term "lanthanide contraction" can be applied to the transition metals of periods 4 (blue), 5 (orange) and 6 (gray). 5th period elements have a larger radius than 4th period elements in the same group, which can be rationalized by having outer electrons in the 5th rather than the 4th shell (e.g. Ni vs Pt).

Perhaps surprisingly, the 6th period elements have about the same radius as the 5th period elements in the same group (e.g. Pt vs Pd). This is explained by the lanthanide contraction (increase in number of protons with inefficient shielding by the increase in f-electrons).

You can see how efficient different electrons are in shielding by looking at the slope of the decrease in atomic radii in the main group, transition metal and f-block group parts of the graph.