what is diagonal relationship ....???explain its causes .......????

This is due to the fact that on moving across a period from left to right electropositivity of the atom decreases.On the other hand on moving down the group electropositivity increases. These two factors work in opposite direction. Hence the elements that are present diagonally to each other have similar properties.

As a result of diagonal relationship the physical properties such as the atomic size, electronegativity, density etc. and chemical properties of compounds such as reaction with water, nitrogen, decomposition reactions etc. are similar.

Thanks.