Question

Describe the origin of the diagonal relationship between Li and Mg.

Answer 1

Diagonal relationship refers to the similarities in the chemical properties of the diagonally placed elements in the second and third period of the periodic table. Due to diagonal relatioship, Li and Mg shows similarities in their properties. Examples for other diagonal pairs are Be and Al, B and Si etc.

Diagonal relationship is due to similar atomic and ionic radii, similar electronegativities similar polarizing power and similar chemical properties between a diagonal pair. When we move across a period, atomic and ionic radii decreases, electonegativity increases, covalent character increases, metallic character decreases and reducing power decreases. Wen moving down a group all these trends are reversed. Down the group size increases, electronegativity decreases, ionic character increases, metallic character increases and reducing power increases. These opposing effects cancels out when we simultaneously move down a group and crosses an element across a period in the periodic table. As a result, diagonally placed elements appear to have similar size, electronegativities and polarising power. The diagonal relationship is more pronounced in the second and third period of the periodic table.

The second and third row has vaccant d orbitals. This make the diagonal relationship more pronounced in the second and third row of the periodic table.

The following are some of the similarities between Li and Mg

1) Both Li and Mg form weak bases - Li(OH) and Mg(OH)2

2) When heated in nitrogen atmosphere, both forms ionic nitrides

6 Li + N2 $\rightarrow$ 2Li3N

3 Mg + N2 $\rightarrow$ Mg3N2

3) Hydroxides of both Mg and Li decomposes on heating

2 LiOH $\rightarrow$ Li2O + H2O

Mg(OH)2 $\rightarrow$ MgO + H2O

4) Both Li and Mg forms monoxides reacting with oxygen while other members in the respective groups form peroxides and superoxides

4 Li + O2 $\rightarrow$ 2Li2O

2 Mg + O2 $\rightarrow$ 2MgO

Li2O and MgO formed does not combine with further oxygen to give peroxides and superoxides

5)Both Li and Mg does not form solid carbonates

6) Carbonates of Li and Mg decomposes on heating to give corresponding oxides by liberating CO2

Li2CO3 $\rightarrow$ Li2O + CO2

MgCO3 $\rightarrow$ MgO + CO2

7) Nitrates of Li and Mg decomposes on heating producing nitrogen oxide

4 LiNO3 $\rightarrow$ 2Li2O + 4NO2 + O2

Mg(NO3)2 $\rightarrow$ MgO + 4NO2 +O2

8) Hydroxides, carbonates, phosphates, oxalates and fluorides of Li and Mg are sparingly soluble in water.

9) Due to high covalent character, Li and Mg chlorides and perchlorates Liare soluble in water

10) LiCl and MgCl2 are deliquescent and crystallise from aqueous solution as hydrate LiCl.2H2O and MgCl2.6H2O