Question

What is the cause of diagonal relationship in elements ?

Answer 1

In a periodic table, an element of a group in the second period is similar in properties with the second element of next group. This type of relation is called Diagonal relationship
Second period    Li      Be        B      C
Third period       Na     Mg       Al        Si
Li-Mg
Be-Al
B - Si
Reasons Similar atomic size or ionic sizesimilar electronegativitySimilar polarising power.Diagonally related elements have same chemical properties.