NEET 2021 Chemistry Question 55 Solution

The steady decrease in atomic and ionic radii along the actinoid series (actinoid contraction) is larger from element to element than lanthanoid contraction. This arises because $5f$ electrons have more diffuse radial wavefunctions and exert even poorer electrostatic shielding of nuclear charge than $4f$ electrons. Statement (1) is scientifically correct.

According to NCERT Chapter on d- and f-Block Elements: *"Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of partially filled f electrons ($f$-$f$ transitions)."* Only ions with an empty subshell ($La^{3+}: 4f^0$) or a completely filled subshell ($Lu^{3+}: 4f^{14}$) are colorless. The vast majority ($Pr^{3+}, Nd^{3+}, Sm^{3+}, Eu^{3+}, Gd^{3+}, Tb^{3+}, Dy^{3+}, Ho^{3+}, Er^{3+}, Tm^{3+}$) possess characteristic vivid colors (e.g., $Nd^{3+}$ is pink, $Pr^{3+}$ is green). Therefore, Statement (2) is demonstrably incorrect.

- Statement 3: Lanthanoids are silvery-white metals and good conductors of heat and electricity (NCERT fact, correct). - Statement 4: Actinoids are highly electropositive and reactive metals, readily attacked by boiling water and oxygen, especially when in finely divided state (NCERT fact, correct).