Question

Knowing that the chemistry of lanthanoids (Ln) is dominated by its+3 oxidation state, which of the following statements is incorrect? (a) The ionic sizes of Ln (III) decrease in general with increasing atomic number. (b) Ln (III) compounds are generally colourless. (c) Ln (III) hydroxides are mainly basic in character. (d) Because of the large size of the Ln (III) ions the bonding in its compounds is predominantly ionic in character.

Short answer

Statement (b) is incorrect as many Ln (III) compounds are colored, not colorless.

Step-by-step solution

Understanding the Oxidation State

Lanthanoids typically exhibit a +3 oxidation state in their compounds. This is a key feature and is necessary to consider when analyzing the properties and behaviors of their compounds.

Lanthanide Contraction

Statement (a) addresses the ionic sizes of Ln (III) ions. Due to the lanthanide contraction, as we move across the lanthanoid series from La to Lu, the ions generally decrease in size with an increase in atomic number. Thus, statement (a) is correct.

Color of Lanthanoid Compounds

Statement (b) claims that Ln (III) compounds are generally colorless. However, many lanthanoid ions can absorb visible light, leading to colored compounds. Therefore, this statement is potentially incorrect as it generalizes too broadly.

Basic Nature of Hydroxides

Statement (c) states that Ln (III) hydroxides are mainly basic. Lanthanoid hydroxides are indeed basic, reacting readily with acids to form salts. This aligns with the usual chemistry of lanthanoids, making statement (c) correct.

Ionic Bonding Character

Statement (d) mentions that due to the large ionic sizes, Ln (III) ions form predominantly ionic bonds in their compounds. This is consistent with the chemical behavior of lanthanoids. Hence, statement (d) is correct.

Key concepts

Lanthanide Contraction

Lanthanoids, sometimes known as rare earth elements, exhibit a unique phenomenon called "lanthanide contraction." As you move from Lanthanum (La) to Lutetium (Lu) across the periodic table, their atomic numbers increase. An interesting effect is observed: the ionic radii of the lanthanide series decrease. This occurs despite the fact that additional electrons are being added. The decrease in size is due to the poor shielding effect of the 4f-electrons. Thus, the nuclear charge experienced by the outer electrons increases, drawing them closer to the nucleus and causing a reduction in size.
This lanthanide contraction explains various properties such as the small differences in radii and ionization energies within the series. As a result, chemical properties tend to be more uniform, making chemical separation of these elements quite challenging. Understanding this concept helps explain why the trends seen in lanthanoids are not always the same as in other elements.

Oxidation States

The +3 oxidation state is the most common for lanthanoids. This oxidation state arises when three electrons are lost, usually ones from the 6s and 4f sublevels. This is because removing these electrons provides a more stable electronic configuration.
While the +3 state prevails, some lanthanoids can exhibit other oxidation states, such as +2 or +4, but these are less stable and less common. These variations in oxidation states can influence color, reactivity, and the magnetic properties of the compounds they form. For instance, compounds with mixed oxidation states might show different physical and chemical behaviors like differing colorations due to electronic transitions between different states within an ion.

Ionic Bonding

Lanthanide compounds largely exhibit ionic bonding, a characteristic that arises from the size and charge of the Ln(III) ions. These large ions tend to lose electrons easily, forming positive ions that can engage in strong attraction with negative ions or anions.
In ionic compounds, the electrostatic attractions between oppositely charged ions provide stability. Thus, many of these lanthanoid compounds are ionic solids that often dissolve in water, yielding electrically conductive solutions. However, due to differences in the degree of ionic vs. covalent character, not all lanthanoid bonds are purely ionic; sometimes, a partial covalent character can be present, contributing to unique variations in compound properties.

Basicity of Hydroxides

Lanthanoid hydroxides are mostly basic, meaning they can easily react with acids to neutralize them and form salts. This basicity is largely due to the presence of the hydroxide ion (OH⁻), which readily accepts protons (H⁺) from acids.
Typically, the base strength can vary among different lanthanoids. For example, hydroxides of lighter lanthanoids tend to be more basic than those of heavier ones. This trend is influenced by the lanthanide contraction which affects the ionic radii and, therefore, the solubility and reactivity of these hydroxides. Understanding the basicity of these hydroxides provides insight into their potential applications in industrial and chemical processes where neutralization of acids is required.