Lithium shows diagonal relationship with element ' X ' and aluminum with Y . X and Y respectively are

 A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods (first 20 elements) of the periodic table.These pairs lithium and magnesium, beryllium and aluminum, boron and silicon, etc.). These similarities in the properties of diagonally adjacent elements are due to the similar polarizing power, similar ionic charge to size ratio of diagonally adjacent elements.
Hence, lithium shows diagonal relationship with magnesium and aluminium shows diagonal relationship with beryllium.