Problem 148 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is true about beryllium? (a) \(\mathrm{Be}(\mathrm{OH})_{2}\) is basic in nature only. (b) Beryllium halides are electron deficient. (c) Aqueous solution of \(\mathrm{BeCl}_{2}\) is acidic. (d) It forms unusual carbide \(\mathrm{Be}_{2} \mathrm{C}\).

Step-by-Step Solution

Answer

Options (b), (c), and (d) are true about beryllium.

Step 1: Understanding Beryllium Hydroxide

Beryllium hydroxide, \( \mathrm{Be(OH)}_2 \), can behave as an acid as well as a base. This dual behavior is known as amphoterism. Therefore, it is not only basic, making option (a) incorrect.

Step 2: Examining Beryllium Halides

Beryllium halides, such as \( \mathrm{BeCl}_2 \), are known to form polymeric structures due to their electron deficiency. This is because the electrons in beryllium are used in forming bonds, and the beryllium atom lacks sufficient electrons to complete its octet, making option (b) correct.

Step 3: Investigating Aqueous \( \mathrm{BeCl}_2 \)

When \( \mathrm{BeCl}_2 \) is dissolved in water, it forms an acidic solution. This happens because \( \mathrm{Be}^{2+} \) is a small, highly charged ion, that draws away electron density from water molecules, leading to the release of \( \mathrm{H}^+ \) ions, confirming option (c) as correct.

Step 4: Analyzing Beryllium’s Unusual Carbide

Beryllium forms an unusual carbide, \( \mathrm{Be}_2 \mathrm{C} \), which is characterized by a unique structure compared to typical metal carbides. This supports option (d) as true.

Key Concepts

Amphoterism

Beryllium hydroxide \( \mathrm{Be(OH)}_2 \) is an interesting molecule due to its ability to behave both as an acid and a base. This property is called amphoterism. It means that \( \mathrm{Be(OH)}_2 \) can react with both acids and bases to form different compounds.
This dual reaction nature places beryllium hydroxide in a unique category. Typically, in solutions:

• With acids, it acts like a base neutralizing the acid.
• With bases, it behaves like an acid reacting with the base.

This property makes \( \mathrm{Be(OH)}_2 \) flexible in its chemical interactions, underlying its amphoteric characteristic.

Electron Deficiency

Beryllium halides, such as \( \mathrm{BeCl}_2 \), are known to be electron-deficient compounds. This occurs because beryllium only has two electrons to share in bonding, limiting its ability to fill its outer shell to an octet.
As a result, beryllium tends to form polymeric structures to compensate for this deficiency. In these structures:

• Each beryllium atom shares electrons with multiple halide ions (like chlorine) extending the structure.
• This sharing helps stabilize the compound, even though each beryllium atom lacks a complete octet individually.

This electron deficiency is a key point in understanding beryllium chemistry, distinguishing it from many other elements.

Acidic Solution

When \( \mathrm{BeCl}_2 \) is dissolved in water, the resulting solution is acidic. This is primarily due to the highly charged \( \mathrm{Be^{2+}} \) ion. A small but highly charged ion can have a strong effect on the surrounding environment.
The positive charge on \( \mathrm{Be^{2+}} \) attracts electrons from water molecules, disrupting the water's structure. This:

• Causes water molecules to lose \( \mathrm{H^+} \) ions (protons), decreasing the solution's pH.
• Results in the release of these \( \mathrm{H^+} \) ions into the solution, giving it an acidic characteristic.

Understanding this effect is important as it plays a role in beryllium’s interactions in aqueous environments.

Unusual Carbides

Beryllium forms an interesting type of carbide known as \( \mathrm{Be}_2 \mathrm{C} \). Unlike typical carbides, \( \mathrm{Be}_2 \mathrm{C} \) does not have the usual characteristics.
This compound is constructed with a unique structure that further differentiates it from more common metal carbides. With \( \mathrm{Be}_2 \mathrm{C} \):

• The structure is not what you might predict when imagining carbide compounds which often have simple, straightforward arrangements.
• It offers insights into the peculiar nature of beryllium's bonding capabilities and behavior.

This uniqueness makes \( \mathrm{Be}_2 \mathrm{C} \) noteworthy in the study of beryllium and its compounds.