Question

Be and Al have following resemblance due to diagonal relationship:
A. have nearly equal electronegativity
B. form amphoteric oxides
C. have same charge/radius ratio
D. both form dimeric halides

Answer 1

Diagonal relationship is a relationship which occurs between some elements of the periodic table, majorly first twenty elements. it occurs in the diagonal form in certain elements such as Lithium and Magnesium, Boron and Silicon etc. It occurs due to the presence of some similar properties.
It is also because some similar properties are seen within Group 1 and Group 2 and so on. The reason can also be traced back to the time when periodic tables were made according to the atomic weights and resulted in the same properties.

Complete step by step answer:
The more scientific reason why the diagonal relationship occurs is because of:
The electronegativities of Al is 1.61 and Be is 1.57 which is almost similar. Hence, option A is correct as they have nearly equal electronegativity.
The oxides which are BeO and $A{{l}_{2}}{{O}_{3}}$ are amphoteric as they can be dissolved in both acids and bases.
$BeO+2HCl\to BeC{{l}_{2}}+{{H}_{2}}O$
$BeO+2NaOH\to N{{a}_{2}}Be{{O}_{2}}+{{H}_{2}}O$
$A{{l}_{2}}{{O}_{3}}+6HCl\to 2AlC{{l}_{3}}+3{{H}_{2}}O$
$A{{l}_{2}}{{O}_{3}}+2NaOH\to 2NaAl{{O}_{2}}+{{H}_{2}}O$

Above equations state that both the oxides dissolve in acids and bases as well.
Aluminium and Beryllium have the same charge/radius ratio because they both have approximately the same size and charge.
Dimeric halides are the one where the same type of element is present and in amount of 2 then, it is 2. Aluminium forms $AlC{{l}_{3}}$ and Beryllium forms $BeC{{l}_{2}}$ where the chlorine is forming the bond more than 2 times. So, the correct answer is “Option A,B,C and D”.

Note: There are some other properties which are similar to beside the above-mentioned points.
The halides of both the elements such as Be and Al act as Lewis Acids and are used in Friedel-Crafts as catalysts.
Carbides of both the elements react with water to release methane gas.