Diagonal relationship between Beryllium and Aluminium

Consider a class of students wherein two kids, apparently not related to each other through bloodlines, share strikingly similar characteristics, which stand as a class apart. The best way to explain the diagonal relationship is by using this analogy between beryllium and aluminium in the periodic table. Although beryllium, marked by the symbol Be, and aluminium, marked by the symbol Al, come from Group 2 and Group 13, respectively, they share some similarities in their chemical properties that are deviations from what would be expected in their respective groups.

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General Concepts and Definitions

The diagonal relationship is the relationship between pairs of elements that are diagonal across each other in the table of elements. It goes from the second period to the third period. This phenomenon could easily be best described by the conduct of beryllium and aluminium, as they are of approximately the same electronegativities and have values for ionization energy that are nearly the same, while the atomic radii are also the same. These two elements combined are in separate groups but possess a very similar chemical behaviour. Beryllium has the atomic number 4 and is one of the light metals, also placing it in Group 2: alkaline earth metals. Due to the balancing ability of elements, while they move diagonally across the periodic table, aluminium bears an atomic number of 13 and appears just below boron in Group 13 of the periodic table.

Various Aspects and Examples

Several aspects of beryllium and aluminium are the same, such as being amphoteric, the formation of covalent compounds, and resistance to acid attacks. The two elements form an oxide layer on their surface that offers them protection from corrosion. As such, beryllium hydroxide, Be(OH)2, and aluminium hydroxide, Al(OH)₂, form amphoteric oxides; thus, they react with both acids and bases. The property is very crucial for various industrial uses of the two elements. Also, beryllium and aluminium combine to form some covalent compounds with certain elements. For instance, beryllium chloride, BeCl₂, and aluminium chloride, AlCl₂, are formed, which presents yet another piece of evidence of the chemical similarity between these elements.

The ionic radius of Be²⁺ is estimated to be 31 pm; the charge/radius ratio is nearly the same as that of the Al³⁺ ion. Hence beryllium resembles aluminium in some ways. Some of the similarities are:

• Like aluminium, beryllium is not readily attacked by acids because of the presence of an oxide film on the surface of the metal.
• Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)₄]^2-just as aluminium hydroxide gives aluminate ion, [Al(OH)₄]^2-
• The chlorides of both beryllium and aluminium have a Cl^– bridged chloride structure in a vapour phase. Both the chlorides are soluble in organic solvents and are strong Lewis acids. They are used as Friedel Craft catalysts.
• Beryllium and aluminium ions have a strong tendency to form complexes, BeF₄^2–, AlF₆^3–.

Relevance and Applications

Some of the important implications of the diagonal relationship between beryllium and aluminium in theory and applications are that it helps students and researchers in academics identify and explain the diverse properties of these and other elements.

• For example, the amphoteric nature of beryllium and aluminium compounds is applied to qualitative methods of analysis and separation techniques in analytical chemistry.
• The uniqueness of beryllium is its application in both the aerospace and military industries for its high strength-to-weight ratio and good thermal stability in industrial applications.
• Aluminium is a more abundant and hence reasonably priced metal, usage of which is in greater quantity in construction, packaging, and transport.
• Aluminium has capability to form protective oxide films rendering both of these metals important materials where good resistance to corrosion is required.

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Some Solved Examples

Example 1

Question: Be and Al show a diagonal relationship, hence both have:

1. Amphoteric nature of oxides
2. Polarising nature
3. The same degree of electronegativity
4. All the above

Solution:

The oxides of both Be and Al are amphoteric in nature. The polarising power of Be²⁺ and Al³⁺ ions is high. Consequently, the compounds develop a covalent nature. The electronegativity values of both elements are the same.

Hence, the answer is option (4) - All the above.

Example 2

Question:

Be and Al exhibits many properties which are similar. But the two elements differ in:

1. Forming covalent compounds
2. Forming polymeric compounds
3. Exhibiting maximum covalency in compounds
4. Exhibiting amphoteric nature in their oxides

Solution:

Be shows a maximum covalency of four while Al shows a maximum covalency of six.

Hence, the answer is option (3) - Exhibiting maximum covalency in compounds.

Example 3

Question:

Beryllium shows a diagonal relationship with:

1. Mg
2. Al
3. B
4. Na

Solution:

Be shows a diagonal relationship with Al.

Hence, the answer is option (2) - Al.

Conclusion

The diagonal relationship between beryllium and aluminium has this exciting interplay with periodic trends that casts them into almost similar chemical properties despite being placed in different groups. How these elements can warrant such similarities through an analysis of their definitions, key properties, and examples drawn from applications in real-life scenarios will be discussed. This will not only enrich our knowledge of chemistry but also have practical implications for various industries. This periodic system is so great that, through diagonal relationships, it offers definite further study and probing of chemistry. Therefore, all rules of the periodic table are mutual and supported by other rules.