Diagonal relationship is shown by certain elements of period.
2nd and 3rd
Group1 Group2 Group3 Group4
IIPeriod Li Be B C
↘ ↘ ↘
IIIPeriod Na Mg Al Si
Certain elements of II period show similarity with their diagonal elements in III period.
They show diagonal relationship, Li resembles Mg, Be resembles Al as these pairs of elements have almost identical ionic radius and polarising power. (i.e. charge/size ratio).