Why do some elements such as li - mg-be-Al have diagonal relationship or behave similarly

Why do some elements such as li - mg-be-Al have diagonal relationship

1.	Why do some elements such as li - mg-be-Al have diagonal relationship or behave similarly
Answer» TION:A Diagonal Relationship is said to EXIST between certain pairs of diagonally adjacent elements in the second and third periods of the PERIODIC table. These pairs (Li & Mg, Be & Al, B & Si etc.) ... Such a relationship occurs because crossing and descending the periodic table have opposing effects.