Animated Solution for Chemistry - d and f-Block Elements: In context of the lanthanoids, which of the following statements is not correct?

Contraction, Oxidation States, and Separation

The lanthanoid series, often tucked away at the bottom of the periodic table, holds some of the most fascinating chemical behaviors in inorganic chemistry. Let's embark on a journey to evaluate the given statements and uncover the truth about these intriguing elements.

Let's look at the first statement regarding atomic radii. As we move across the lanthanoid series from Cerium to Lutetium, electrons are progressively added to the inner $4f$ subshell.

These $4f$ electrons are notorious for their poor shielding effect. Because they are deeply buried and have a diffused shape, they fail to effectively shield the outer electrons from the increasing nuclear charge. As a result, the nucleus pulls the outermost shell closer, leading to a steady and gradual decrease in both atomic and ionic radii. This phenomenon is famously known as Lanthanoid Contraction. Therefore, the first statement is absolutely correct.

Moving on to the second statement, it claims that all members exhibit a $+3$ oxidation state. This is a hallmark characteristic of the lanthanoids.

The energy required to remove the first three electrons (the sum of the first three ionization enthalpies) is relatively low and is easily compensated by the high hydration energy or lattice energy when they form compounds. Thus, the $+3$ oxidation state is the most common and stable state for the entire series, making the second statement perfectly true.

The third statement touches upon the difficulty of separating lanthanoids. Imagine trying to sort identical twins; that is exactly what chemists face here.

Because of the lanthanoid contraction, the decrease in ionic radii across the series is incredibly small. This nearly identical size means their chemical properties are virtually indistinguishable. In nature, they are always found together in the same ores, and separating them requires tedious techniques like ion-exchange chromatography. Hence, the statement that their separation is not easy is entirely correct.

Finally, we arrive at the fourth statement, which boldly claims that all members form compounds in the $+4$ oxidation state. There is a catch here.

While the $+3$ state is universal, removing a fourth electron requires a massive amount of energy (the fourth ionization enthalpy is exceptionally high). Only a select few elements can achieve this. The most famous example is Cerium (Ce), which readily forms a $+4$ state because losing four electrons leaves it with a highly stable, empty $4f^0$ noble gas configuration (like Xenon).

For the vast majority of the lanthanoids, the $+4$ state is energetically forbidden. Therefore, the claim that all members show a $+4$ state is fundamentally incorrect.

By systematically analyzing the chemistry of the $f$-block, we can confidently conclude that statement (d) is the incorrect one, making it our final answer.