Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

In context of the lanthanoids, which of the follwoing statements is not correct? (a) Availability of 4 f electrons results in the formation of compound in \(+4\) state for all the members of the series. (b) There is a gradual decrease in the radii of the members with increasing atomic number in the series. (c) Because of similar properties the separation of lanthanoids is not easy (d) All the members exhibit \(+3\) oxidation state.

Step-by-Step Solution

Answer

Option (a) is not correct.

Step 1: Understand the Lanthanoids

Lanthanoids, also known as lanthanides, are a series of 15 chemical elements in the periodic table with atomic numbers from 57 to 71, from lanthanum through to lutetium. They are f-block elements, known for their high atomic numbers, and are typically involved in +3 oxidation states among other properties.

Step 2: Analyze Option (a)

Option (a) claims that all lanthanoids can exhibit a +4 oxidation state due to the availability of 4f electrons. However, not all lanthanoids can stabilize this oxidation state. Only a few, such as cerium, can easily form +4 ions. Hence, this statement is generally incorrect.

Step 3: Analyze Option (b)

This option states there is a gradual decrease in the radii of the lanthanoids with increasing atomic number, which is known as the lanthanoid contraction. This is a well-documented phenomenon and is correct.

Step 4: Analyze Option (c)

This option mentions that due to similar properties, separating lanthanoids is not easy. Indeed, lanthanoids have very similar physical and chemical properties, making their separation challenging without advanced techniques. Thus, this statement is correct.

Step 5: Analyze Option (d)

Option (d) states that all lanthanoids exhibit +3 oxidation states. This is the most common oxidation state for lanthanoids due to the stability of having a half-filled or empty f orbital. This statement is correct.

Step 6: Identify the Incorrect Statement

Having analyzed all options, it is clear that option (a) is incorrect. The generalization that all lanthanoids form +4 state compounds is not valid since only a few do.

Key Concepts

Lanthanoid Contraction

Lanthanoids, also known as lanthanides, exhibit an interesting trend in their atomic and ionic radii as their atomic number increases. This trend is called 'lanthanoid contraction.'
In simple terms, lanthanoid contraction refers to the gradual decrease in the size of the lanthanoid ions as we move across the series from lanthanum (La) to lutetium (Lu). This happens despite increasing atomic numbers and additional inner electrons. The reason behind this contraction lies in the poor shielding effect of the 4f electrons. Because these electrons do not efficiently shield the nucleus' positive charge, the outer electrons are pulled closer to the nucleus, leading to a smaller atomic radius. This subtle yet impactful phenomenon significantly affects the chemistry of the entire series.

• It influences the formation of compounds.
• It affects the separation processes of these elements due to size-related factors.
• This contraction even impacts the physical properties, such as a trend in hardness and melting points across the series.

Understanding lanthanoid contraction is crucial for appreciating the nuances of lanthanoid chemistry.

Oxidation States

The lanthanoid series is characterized by its typical +3 oxidation state. This uniformity stems from the electronic configuration of the lanthanoids, typically involving a loss of three electrons to achieve a stable configuration. However, other oxidation states can also be observed in certain elements of the series due to the unpaired 4f electrons.

While +3 is common, some lanthanoids like cerium, praseodymium, and terbium can exhibit a +4 oxidation state. This is because they have electronic configurations that allow them to reach stability by losing one more electron beyond the +3 state. In contrast, lanthanoids like europium and ytterbium can sometimes exist in a +2 state, although this is less common and usually less stable. These variabilities in oxidation states prove useful in various chemical reactions and are crucial for applications ranging from catalysis to material sciences. Recognizing these subtle differences aids in understanding the reactivity and potential uses of each element within the lanthanoid group.

Separation of Lanthanoids

The separation of lanthanoids poses significant challenges due to their remarkably similar chemical and physical properties. Their near-identical ionic radii, conventional oxidation states, and behaviors in chemical reactions make traditional separation techniques, like precipitation or simple chemical reactions, less effective. Advanced separation methods are necessary to achieve effective separation of lanthanoids. These methods often include:

• Ion-exchange chromatography: Relies on subtle differences in the ionic sizes and solubility to separate lanthanoids.
• Solvent extraction: Utilizes different solubilities or complexing agent affinities to separate the elements.
• Fractional crystallization: A method that exploits slight differences in the solubility of lanthanoid salts.

These refined techniques are crucial for obtaining pure samples of each lanthanoid, which can then be used in specialized applications, including electronics, magnetic materials, and other high-tech industries. Understanding this process is key to appreciating the distinct yet collective nature of the lanthanoid series.