Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Beryllium and aluminium exhibit many properties which are similar. But the two elements differ in [2004] (a) forming polymeric hydrides (b) forming covalent halides (c) exhibiting maximum covalency in compounds (d) exhibiting amphoteric nature in their oxides.

Step-by-Step Solution

Answer

The difference between beryllium and aluminum is in forming polymeric hydrides (option a).

Step 1: Understanding Amphoteric Nature

Beryllium oxide \((BeO)\) and aluminum oxide \((Al_2O_3)\) are both amphoteric, which means that they can react with both acids and bases to form salts and water. This step confirms that both elements share this property, so we eliminate option (d) as a differentiating factor.

Step 2: Examining Covalent Halides Formation

Both beryllium and aluminum form primarily covalent halides such as \(BeCl_2\) and \(AlCl_3\). Since this is a common property, option (b) is not a point of differentiation.

Step 3: Investigating Polymerization in Hydrides

Beryllium forms polymeric hydrides like \((BeH_2)_n\) whereas aluminum does not form polymeric hydrides. This step indicates a difference between the two elements, thus we focus on option (a) as a potential answer.

Step 4: Analyzing Maximum Covalency

Aluminum can show a maximum covalency of 6, as seen in complex ions like \([AlF_6]^{3-}\), but beryllium typically does not exceed a covalency of 4. This difference suggests option (c) may also be correct.

Key Concepts

Amphoteric Nature of Oxides

The amphoteric nature of an oxide speaks to its ability to react both with acids and bases. This attribute allows the oxide to act as either a base or an acid depending on its chemical surroundings. Beryllium oxide \[(\text{BeO})\] and aluminum oxide \[(\text{Al}_2\text{O}_3)\] are prime examples of amphoteric oxides:

• When reacting with acids, they can neutralize and form salts and water.
• Similarly, they neutralize bases by also forming salts and water.

This dual ability is a shared attribute between these oxides which might explain why it isn't helpful in distinguishing between beryllium and aluminum for differentiating purposes in some exercises. However, understanding this concept is essential because it is a fundamental property of these oxides that influences their behavior in various chemical reactions.

Covalency in Compounds

Covalency refers to a form of chemical bonding where electron pairs are shared between atoms. Elements like beryllium and aluminum often form covalent bonds in their compounds.

• Beryllium often exhibits a lower covalency, commonly not exceeding four.
• In contrast, aluminum can demonstrate a maximum covalency of six, such as in the complex ion \([\text{AlF}_6]^{3-}\).

The difference in maximum covalency is significant because it reveals aluminum's ability to engage in more extensive bonding networks than beryllium. This property is important to note when predicting the compound formation capabilities of these elements.

Polymeric Hydrides

In chemistry, polymeric hydrides refer to compounds where hydrogen atoms link together with a metal in a repeating unit structure or polymer. Beryllium exhibits this property by forming polymeric hydrides such as \((\text{BeH}_2)_n\).

• These hydrides consist of chains where the hydrogen atoms form bridges between beryllium centers.
• This bridging stabilizes the hydride structure, resulting in polymerization.

Unlike beryllium, aluminum does not tend to form polymeric hydrides, making this a distinguishing feature between the two elements. Understanding polymeric hydrides is crucial as it influences the material's properties and behavior in various chemical contexts.