Diagonal Relationship of Li with Mg
Due to its small size lithium differs from other alkali metals but resembles to Mg as its size is closer to Mg Its resemblance with Mg is known as diagonal relationship. Usually the periodic properties shows either increasing or decreasing trend along group and vice versa along the period which brought diagonally situated elements to the closer values. Following are the characteristic which should be noted.
Period Group I Group II
2 Li Be
3 Na Mg
(1) Li and Mg both are harder and higher melting point than the other metals of their groups.
(2) Because of covalent nature, chlorides of Li and Mg both are deliquescent and soluble in alcohol and pyridine while the chlorides of other alkali metals are not so.
(3) The fluorides, phosphates of Li and Mg are sparingly soluble in water whereas those of the other alkali metals are soluble in water.
(4) The carbonates of Li and Mg decompose on heating and liberate CO2 Carbonates of the other alkali metals are stable towards heat and decomposed only on the fusion.
Li2CO3 → Li2O + CO2; Mg CO3 → MgO + CO2
(5) Hydroxides and the nitrates of both Li and Mg decompose on heating to provide 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 the other alkali metals are stable towards heat while their nitrates provide O2 and nitrite.
2KNO3 → 2KNO2 + O2
(6) Both Li and Mg combine directly with N2 to give nitrides Li3N and Mg3N2. The other alkali metals combine at the high temperature,
6Li + N2 → 2Li3N; 3Mg + N2 → Mg3N2.
Both nitrides are decomposed by the water to provide NH3
Li3N + 3H2O → 3LiOH + NH3 ;
Mg3N2 + 6H2O → 3Mg(OH)2+ 2NH3
(7) The bicarbonates of Li and Mg are much more soluble in water than carbonates whereas carbonates of alkali metals are more soluble.
(8) Both Li and Mg combine with carbon on heating.
2Li + 2C → Li2C2 ; Mg + 2C → Mg C2
(9) Periodic properties of the Li and Mg are very much 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
(10) Both have high polarizing power.
Polarizing Power = Ionic charge / (ionic radius)2.
(11) Li and Mg Form only monooxide on heating in oxygen.
4Li + O2 → 2 Li2O ; 2Mg + O2 → 2 MgO
(12) Li2SO4 like MgSO4 does not form alums.
(13) The bicarbonates of Li and Mg do not exist in solid state, they are present in solution only.
(14) Alkyls of Li and Mg (R. Li and R.MgX) are soluble in the organic solvent.
(15) Lithium chloride and MgCl2 both are deliquescent and separate out from their aqueous solutions as hydrated crystals, LiCl. 2H2O and MgCl2 . 2H2O.
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