The f-block elements are those in which the differentiating electrons enters the (n-2)f orbitals. There are two series of f-Block elements corresponding to filling of 4f and 5f-orbitals. The series of 4f-orbitals is called lanthanides. Lanthanides show different oxidation states depending upon stability of f⁰, f⁷ and f¹⁴ configurations, though the most common oxidation states is +3. There is a regular decrease in the size of lanthanides ions with increase in atomic number which is known as lanthanides contraction.

Lanthanide contraction is observed in which of the following?

Gd

The correct answer is option 1. Gd.

Lanthanide contraction refers to the phenomenon where there is a gradual decrease in the atomic and ionic radii of the lanthanide series elements as you move from lanthanum (La, Z=57) to lutetium (Lu, Z=71). This contraction is primarily due to the poor shielding effect of the 4f electrons, which results in an increased effective nuclear charge felt by the outermost electrons.

Let us analyze the given options:

1. Gd (Gadolinium): Gadolinium is a lanthanide element (Z=64). It is indeed part of the lanthanide series and exhibits lanthanide contraction.

2. At (Astatine): Astatine is a halogen element (Z=85) and is not part of the lanthanide series. It does not exhibit lanthanide contraction.

3. Xe (Xenon):Xenon is a noble gas element (Z=54) and is not part of the lanthanide series. It does not exhibit lanthanide contraction.

4. Te (Tellurium): Tellurium is a metalloid element (Z=52) and is not part of the lanthanide series. It does not exhibit lanthanide contraction.

Conclusion: The only element among the options given that exhibits lanthanide contraction is Gd (Gadolinium). Therefore, the correct answer is Gd