Anomolous Behaviour of Lithium and its Diagonal Relationship with Magnesium

The properties of lithium are quite different from the properties of other alkali metals. On the other hand, it shows greater resemblance with magnesium, which is diagonally opposite element of it. The main reasons for the anomalous behaviour of lithium as compared to other alkali metals are

(i)     The extremely small size of lithium atom and its ion.

(ii)    Greater polarizing power of lithium ion ( Li+), due to its small size which result in the covalent character in its compounds.

(iii)    Least electropositive character and highest ionization energy as compared to other alkali metals.

(iv)   Non availability of vacant d-orbitals in the valence shell.
 

The reason for resemblance of properties of lithium with magnesium is that these two elements have almost same polarizing power.

The following points illustrate the anomalous properties of lithium and its diagonal relationship with magnesium:

(a)    The melting point and boiling point of lithium are comparatively high.

(b)    Lithium is much harder than the other alkali metals. Magnesium is also hard metal.

(c)    Lithium reacts with oxygen least readily to form normal oxide whereas other alkali metals form peroxides and superoxides.

(d)   LiOH like Mg (OH)2  is weak base. Hydroxides of other alkali metals are strong bases.

(e)    Due to their appreciable covalent nature, the halides and alkyls of lithum and magnesium are soluble in organic solvents.

(f)     Unlike elements of group 1 but like magnesium. Lithium forms nitride with nitrogen.

6Li + N2 → 2Li3N       

(g)    LiCl is deliquescent and crystallizes as a hydrate, LiCI2H2O. Other alkali metals do not form hydrates.  also forms hydrate, MgCI2.8H2O .

(h)    Unlike other alkali metals lithium reacts directly with carbon to form an ionic carbide. Magnesium also forms a similar carbide.

(i)     The carbonates, hydroxides and nitrates of lithium as well as magnesium decompose on heating.

Li2CO3 → Li2O + CO2       

MgCO3 → MgO + CO2               

2LiOH → Li2O + H2O               

Mg (OH)2  → MgO + H2O             

4LiNO3 → 2Li2O + 4NO2 + O2               

2Mg ( NO3)2 → 2Mg + 4NO2 +O2               

The corresponding salts of other alkali metals are stable towards heat.

 

(j)     Lithium nitrate, on heating, decomposes to give lithium oxide, Li2O  whereas other alkali metals nitrate decomposes to give the corresponding nitrite.

4LiNO3 → 2Li2O + 4NO2 + O2  

2NaNO3 → 2NaNO2 + O2               

2KNO3 → 2KNO2 + O2               

               

(k)  Li2CO3, LiOH, LiF and Li3PO4   are the only alkali metal salts which are insoluble in water. The corresponding magnesium compounds are also insoluble in water.

 

(l)     Hydrogen carbonates of both lithium and magnesium can not be isolated in solid state. Hydrogen carbonates of other alkali metals can be isolated in solid state.

 
Illustration:

        Name the chief factor responsible for the anomalous behaviour of lithium.

Solution:

The chief factors responsible for the anomalous behaviour of lithium are:

(i)     Its very small size,

(ii)    High electronegativity,

(iii)    High ionization enthalpy and

(iv)   Absence of vacant d-atomic orbital in the valence shell

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