Mention any three diagonal relationship similarities between Beryllium and Aluminium.

Hint:Diagonal relationship is said to exist between some certain pair of diagonally adjacent elements in the second and third periods of the periodic table. These pairs show similar characteristics and have similar properties.

Complete step by step answer:
Diagonal relationship exists between certain pairs of diagonally adjacent elements in the second and third periods of the periodic table. This is because as we go down the group atomic size increases and as we go left to right in a period the atomic size decreases. As a result, sometimes these properties cancel each other out and an element of the second period has a size similar to the diagonally adjacent element of the third period. So, these pairs of elements show similar properties.
The similar properties between such a pair of Beryllium and Aluminium are:
1.Beryllium and Aluminium both have nearly similar electronegativity and their charge/size ratio is also similar. They also prefer covalent bonding rather than ionic bonds.
2.Beryllium and Aluminium both are fairly resistant to the action of acids. This is because they form a protective layer/film of oxide on the surface. Beryllium forms ${\text{BeO}}$ while Aluminium forms ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ .
3.Both the metals react with strong alkalis to form soluble complexes. Beryllium forms ${{\text{[Be(OH}}{{\text{)}}_{\text{4}}}{\text{]}}^{{\text{2 - }}}}$ . Aluminium forms ${{\text{[Al(OH}}{{\text{)}}_{\text{4}}}{\text{]}}^{\text{ - }}}$ .

Note:
Some of the other pairs which show diagonal relationship are: Lithium-Magnesium and Boron-Silicon. These elements are also called bridge elements. The other periods of the periodic table do not show diagonal relationship due to the presence of extra orbitals like d and f-orbital due to which the size increase and size decrease effect do not cancel each other out.