Q.Which of the following pairs of elements exhibit diagonal relationship?

Certain second-period elements resemble the third-period element diagonally adjacent to them (one period down, one group right) more closely than they resemble the element directly below them in their own group -- this is the diagonal relationship, seen classically in Li-Mg, Be-Al, and B-Si.

When moving across a period, atomic/ionic size decreases and electronegativity/ionising power increases; when moving down a group, atomic/ionic size increases and electronegativity/polarising power decreases. Moving diagonally (one step down, one step right) roughly cancels these two trends, so a period-2 element and the period-3 element diagonally below-right of it end up with similar ionic size and similar charge/radius ratio (polarising power), giving them comparable chemical behaviour.

Classic examples:

1. Lithium (Li) and Magnesium (Mg): both form normal oxides (not peroxides) on burning in air/oxygen; both form covalent nitrides; their carbonates, on heating, decompose to give the oxide and CO2; both form covalent alkyl/aryl compounds (organometallics); both LiCl and MgCl2 are soluble in ethanol/organic solvents.
2. Beryllium (Be) and Aluminium (Al): both are amphoteric (react with both acids and alkalis); both have similar high charge density/small size giving largely covalent compounds; both form complex ions like BeF4(2-) and AlF6(3-); oxides of both (BeO, Al2O3) are amphoteric and have high melting points. …