Q.Which one of the following elements shows diagonal relationship with magnesium?

Lithium (top-left, Group 1) and magnesium (one row down, one group right, Group 2) are similar in size and charge density, so they show the classic diagonal relationship.

The diagonal relationship arises because, moving diagonally down-right across the periodic table, the effects of increasing atomic size (going down a group) and decreasing size (going across a period) partly cancel, leaving elements with similar ionic size/charge density and hence similar chemical behaviour.

Li and Mg share several properties as a result:

• Both form predominantly covalent compounds among their respective groups.
• Both carbonates decompose on heating to give the oxide + $CO_2$ (unlike other alkali metal carbonates, which are thermally stable). …