Q.What is diagonal relationship? What are the similarities between the properties of Li and Mg?

Diagonal relationship: 2nd-period elements resemble the 3rd-period element diagonally next to them due to similar size/charge density; Li resembles Mg this way.

Diagonal relationship: Certain elements of the second period show close similarities in properties (more than with other members of their own group) to elements placed diagonally below-and-to-the-right of them in the third period — e.g., Li (Group 1) with Mg (Group 2), Be (Group 2) with Al (Group 13), and B (Group 13) with Si (Group 14). This happens because, moving diagonally, the increase in atomic/ionic size (going down a group) is roughly offset by the decrease in size (going across a period), so the diagonal pair ends up with comparable ionic radius and hence similar charge density (polarizing power), giving them comparable covalent character and chemical behaviour.

Similarities between Li and Mg:

1. Both form only the normal oxide (Li2O, MgO) on burning in air/oxygen, unlike other alkali metals (which form peroxides/superoxides).
2. Carbonates of both Li and Mg are thermally unstable and decompose on heating to give the oxide + CO2 (unlike Na2CO3, which is stable to heat).
3. Nitrates of both decompose on heating to give the oxide, NO2, and O2 (unlike other alkali metal nitrates, which decompose to the nitrite). …