Question

Diagonal relationship between lithium and magnesium chemical properties

Answer 1

Diagonal Relationship of Li with Mg

Due to its small size lithium differs from other alkali metals but resembles with Mg as its size is closer to Mg Its resemblance with Mg is known as diagonal relationship. Generally the periodic properties show either increasing or decreasing trend along the group and vice versa along the period which brought the diagonally situated elements to closer values. Following are the characteristic to be noted.

Period

Group I

Group II

2

Li

Be

3

Na

Mg

Both Li and Mg are harder and higher m.pt than the other metals of their groups.

Due to covalent nature, chlorides of both Li and Mg are deliquescent and soluble in alcohol and pyridine while chlorides of other alkali metals are not so.

Fluorides, phosphates of Li and Mg are sparingly soluble in water whereas those of other alkali metals are soluble in water.

Carbonates of Li and Mg decompose on heating and liberate CO2 Carbonates of other alkali metals are stable towards heat and decomposed only on fusion.

Li2CO3 → Li2O + CO2  ; Mg CO3 → MgO + CO2

Hydroxides and nitrates of both Li and Mg decompose on heating to give oxide. Hydroxides of both Li and Mg are weak alkali.

4 LiNO3 → 2Li2O + 4NO2 + O2

2Mg(NO3)2 → 2MgO + 4NO2 + O2

2LiOH → Li2O + H2O ; Mg(OH)2 → MgO + H2O

Hydroxides of other alkali metals are stable towards heat while their nitrates give O2 and nitrite.

2KNO3 → 2KNO2 + O2

Both Li and Mg combine directly with N2 to give nitrides Li3N and Mg3N2. Other alkali metals combine at high temperature, 6Li + N2 → 2Li3N; 3Mg + N2 → Mg3N2. Both the nitrides are decomposed by water to give NH3

Li3N + 3H2O → 3LiOH + NH3;

Mg3N2 + 6H2O → 3Mg(OH)2+ 2NH3

Bicarbonates of Li and Mg are more soluble in water than carbonates whereas carbonates of alkali metals are more soluble.

Both Li and Mg combine with carbon on heating.

2Li + 2C → Li2C2  ; Mg + 2C → Mg C2

The periodic properties of Li and Mg are quite comparable 

 

Li

Mg

Electronegativity

1.0

1.2

Atomic radii

1.23

1.36

Ionic radii

0.60 (Li+)

0.65(Mg+2)

Atomic volume

12.97 c.c

13.97 c.c

 

Both have high polarizing power.
p
Polarizing Power = Ionic charge/(ionic radius)2.

Li and Mg Form only monooxide on heating in oxygen.

4Li + O2 → 2 Li2O ; 2Mg + O2 → 2 MgO

Li2SO4 like MgSO4 does not form alums.

The bicarbonates of Li and Mg do not exist in solid state, they exist in solution only.

Alkyls of Li and Mg (R. Li and R.MgX) are soluble in organic solvent.

Lithium chloride and MgCl2 both are deliquescent and separate out from their aqueous solutions as hydrated crystals, LiCl. 2H2O and MgCl2 . 2H2O.