Question

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

Short answer

Beryllium and aluminium differ in (c) exhibiting maximum covalency in compounds.

Step-by-step solution

Understanding the Problem

Beryllium (Be) and Aluminium (Al) have similar chemical properties due to their diagonal relationship in the periodic table. The task is to find which property differentiates beryllium from aluminium from the given options.

Analyzing the Options

Let's break down each option: (a) Forming polymeric hydrides: Beryllium does not form polymeric hydrides, while aluminium can form complex hydride structures like polymeric hydrides. (b) Forming covalent halides: Both Be and Al form covalent halides. (c) Exhibiting maximum covalency in compounds: Aluminium can exhibit a higher covalency (up to 6 in compounds like AlCl3) compared to beryllium, which exhibits a maximum covalency of 4. (d) Exhibiting amphoteric nature in their oxides: Both metals form amphoteric oxides (BeO and Al2O3).

Selecting the Correct Option

From the analysis, options (b) and (d) cannot be the answers because both beryllium and aluminium exhibit these properties. Option (a) is false for beryllium but true for aluminium. Option (c) accurately reflects that aluminium has a higher covalency than beryllium.

Conclusion

The major property in which beryllium and aluminium differ is the maximum covalency in compounds. Beryllium has a maximum covalency of 4, whereas aluminium can exhibit a maximum covalency of 6.

Key concepts

Diagonal Relationship

The periodic table is structured in a way that includes fascinating relationships between elements. One such relationship is the diagonal relationship. This describes the similarities in properties between elements that are diagonally adjacent to each other in the periodic table. An example is beryllium (Be) and aluminium (Al). These elements are from different groups (the alkaline earth metals and the boron group, respectively) but show striking resemblances in their chemical properties. This is because of their diagonal positioning.

The diagonal relationship occurs due to two main reasons:

• Atomic Size: As you move across the period and down the group, the increase in atomic size is somewhat balanced, leading to comparable atomic and ionic radii.
• Charge Density: The charge to volume ratio is similar between the diagonally related elements, which affects how elements interact chemically.

Therefore, although beryllium and aluminium are part of different families in the periodic table, they exhibit similar chemical behaviour. This diagonal relationship is an essential concept for understanding trends in the periodic table.

Chemical Properties of Beryllium and Aluminium

Beryllium and aluminium, despite being from different groups, share some chemical properties due to their diagonal relationship. However, there are distinctive differences that set them apart. Let's explore both their similarities and their differences.

Similarities:

• Both form covalent halides, such as BeCl₂ and AlCl₃, due to their small size and high ionization energy.
• They both exhibit amphoteric nature in their oxides: BeO reacts with acids and bases, as does Al₂O₃.

Differences:

• Beryllium does not form polymeric hydrides, whereas aluminium is capable of forming complex hydride structures.
• Aluminium can achieve higher covalency in compounds, up to 6, while beryllium is limited to a covalency of 4.

These distinctive properties are crucial for distinguishing between these elements in their chemical behaviour and reactivity.

Covalency in Compounds

Covalency refers to the number of covalent bonds an atom can form with other atoms. This aspect is significant when discussing beryllium and aluminium, as it is where their properties diverge significantly.

Beryllium can exhibit a maximum covalency of 4. This limitation stems from its electronic configuration and its ability to form only a certain number of covalent bonds. Aluminium, in contrast, can have a covalency as high as 6. This is possible because aluminium has a d-orbital that can accommodate extra electrons, allowing it to hybridize and form more bonds.

The implications of different covalency levels are vast. For instance, it affects the types of compounds these elements can form and their respective applications. Aluminium's ability to create a wide variety of compounds due to higher covalency makes it incredibly versatile in chemical reactions and industrial applications. Understanding covalency is crucial for predicting and explaining the chemical behaviour of these elements in compounds.