The lanthanide contraction refers to the gradual decrease in size of the lanthanide series elements, which are the 14 elements from lanthanum (La) to lutetium (Lu). This phenomenon affects the atomic sizes of elements in both the 6th and 7th periods of the periodic table.
As we move across the lanthanide series, from La to Lu, the nuclear charge increases due to the addition of protons. However, the shielding effect provided by the inner f-electrons is not sufficient to completely offset this increase in nuclear charge. As a result, the effective nuclear charge acting on the outer electrons increases, drawing them closer to the nucleus and leading to a reduction in atomic radii.
This contraction in atomic size is crucial when looking at the elements in the 6th period, particularly the transition metals and the heavy main-group elements. For instance, the size of the transition metals from titanium (Ti) to zinc (Zn) does not increase as dramatically as expected; instead, they remain smaller because of the influence of the lanthanide contraction that was already affecting the preceding elements.
When we consider the 7th period, a further contraction occurs due to similar reasons. The presence of the heavier actinides, combined with increasing effective nuclear charge but still the insufficient shielding from f-electrons, results in a surprisingly compact size of the elements, even with the addition of the expected new energy levels.
In summary, lanthanide contraction plays a significant role in determining the atomic radii of both the 6th and 7th periods. It explains why the elements in these periods do not follow the typical trend of increasing size with higher atomic number and indicates how subtle interactions of electronic structure can significantly influence the properties of elements.