Introduction:
Beryllium (Be) and aluminium (Al) are both metallic elements that exhibit several similarities in their properties. However, they differ in terms of their ability to exhibit maximum covalency in compounds.

Explanation:
Covalency refers to the number of covalent bonds that an atom can form in a compound. It is influenced by factors such as the number of valence electrons and the size of the atom. In general, elements tend to exhibit a maximum covalency that is equal to the number of valence electrons they possess.

Beryllium:
Beryllium is a Group 2 element with 2 valence electrons. It is a small-sized atom with a high charge density, which means that its valence electrons are strongly attracted to the nucleus. Consequently, beryllium tends to exhibit a maximum covalency of 4, forming compounds with a tetrahedral arrangement around the central beryllium atom. Examples include BeCl2 and BeF2, where beryllium forms two covalent bonds.

Aluminium:
Aluminium is a Group 13 element with 3 valence electrons. It is a larger-sized atom compared to beryllium, and its valence electrons are less strongly attracted to the nucleus. As a result, aluminium tends to exhibit a maximum covalency of 6, forming compounds with an octahedral arrangement around the central aluminium atom. Examples include AlCl3 and AlF6^3-, where aluminium forms three covalent bonds.

Difference:
The difference between beryllium and aluminium in terms of maximum covalency arises from their atomic size and charge density. Beryllium, being a smaller atom with a higher charge density, can accommodate a maximum covalency of 4. On the other hand, aluminium, being a larger atom with a lower charge density, can accommodate a maximum covalency of 6.

Conclusion:
In summary, beryllium and aluminium differ in their ability to exhibit maximum covalency in compounds. Beryllium can exhibit a maximum covalency of 4, while aluminium can exhibit a maximum covalency of 6. This difference is attributed to their atomic size and charge density.