Problem 147 - A Complete Resource Book in Chemistry for JEE Main

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.

Step-by-Step Solution

Answer

Statement (b) is incorrect.

Step 1: Analyzing Ionic Size

The ionic sizes of the lanthanides decrease across the series because of the lanthanide contraction. This results from the increasing nuclear charge with increasing atomic number, which pulls the electrons closer to the nucleus. Therefore, statement (a) is true.

Step 2: Understanding Color

Lanthanoid compounds in the +3 oxidation state are generally not colorless due to the f-f transitions, even though these transitions are less intense. Thus, statement (b) suggesting Ln (III) compounds are generally colorless is incorrect.

Step 3: Examining Hydroxide Character

Ln (III) hydroxides demonstrate a basic character, consistent with many basic metal oxides and hydroxides. Such compounds often react with acids to form water and a salt. Thus, statement (c) is correct.

Step 4: Evaluating Bonding Character

Lanthanoids have large ionic sizes, leading to predominantly ionic bonding in their compounds. This is typical given the nature of these elements. Therefore, statement (d) is true.

Key Concepts

Oxidation States

Lanthanoids, a series of 15 metallic elements from lanthanum to lutetium, exhibit a notable tendency towards the +3 oxidation state. This is largely because the removal of three electrons results in stable electronic configurations with half-filled or fully filled 4f orbitals. In general, the stability of the +3 oxidation state is due to the high energy required to remove more than three electrons. Consequently, the +3 state predominates in their chemical behavior. However, few elements like europium and cerium can also exhibit +2 and +4 states, respectively, due to additional stability factors. The understanding of oxidation states is essential for recognizing the redox behavior of these elements in various chemical processes.

Lanthanoid Contraction

The phenomenon of lanthanoid contraction refers to the gradual decrease in the ionic radii of lanthanide elements as one moves from lanthanum (Z=57) to lutetium (Z=71) across the periodic table. This occurs despite having a similar charge of the +3 ion. The primary reason for this contraction is the progressive increase in the nuclear charge. Increased nuclear charge pulls electrons more closely towards the nucleus, making the atomic size smaller.
As these elements have similar electronic configurations, it results in observable physical and chemical property variations, such as increased hardness and higher melting and boiling points across the series. Additionally, lanthanoid contraction influences the separation techniques due to the slight differences in chemical reactivity and solubility, making it crucial in industrial applications and the synthesis of complex compounds.

f-f Transitions

One of the key features of lanthanoid elements is their electronic configuration, specifically their 4f electrons, which are responsible for f-f transitions. These transitions refer to the electronic transitions between f-orbitals that give rise to the absorption and emission of visible light. Despite being relatively forbidden and weak due to the shielding effect of outer electrons, these transitions produce a variety of colors. The resulting hues are often subtle and muted compared to more intense d-d transitions seen in transition metals.
The understanding of f-f transitions aids in the study of optical properties and spectral analysis of lanthanoid compounds, proving significant in materials science. For example, it's essential in designing phosphors used in television screens and LED technology, where specific color emissions are required.

Ionic Bonding

Lanthanoids predominantly form ionic compounds due to their large ionic sizes and the +3 oxidation state. These characteristics result in the formation of electrostatic attractions between the positively charged lanthanoid ions and negatively charged counterions.
The nature of ionic bonding in lanthanoid compounds results in high melting points and specific conductivity properties. The bonding is strongly influenced by the lattice energy of the crystals formed, as well as the size and nature of the ions involved.
In practical applications, understanding ionic bonding in lanthanoid chemistry is vital for their use in materials that require robust physical properties. For instance, lanthanoids are critical components in catalysts and advanced ceramics, where ionic bonding enhances functionality and performance under various conditions.