ANOMALOUS BEHAVIOR OF Li AND DIAGONAL RELATIONSHIP WITH Mg:

It is observed for 3 elements of periodic table that they resemble more with their diagonal element than their same group elements. This property is named as Diagonal Relationship. 

Lithium exhibits most of the characteristic properties of the group 1 elements, but it differs in many respects from them and resemble with Magnesium which is Diagonal to it in Periodic Table.

This anomalous behavior of lithium is due to it’s extremely small size it’s ion also.

The small size of the Li⁺ ion leads to its high charge density. Lithium ion, therefore, possesses the greatest polarising power out of all the alkali metal ions. Hence it exerts a great distorting effect on the negative ion. Consequently the Li⁺ ion has remarkable great tendency towards solvation and convalent bond formation. Further it is important to note that the polarising power of the Li⁺ ion is similar to that of Mg²⁺ ion, hence the two elements (placed diagonally in the periodic table) resemble very much in their properties.

      The diagonal relationship (similarities) between the two elements of the periodic table can be explained on the basis of electropositive character and polarising power of the elements.

(i)      The electropositive character of the elements increases as we go down a group but the electropositive character decreases as we go from left to right along a period. Thus results in similarity in the electropositive character of the diagonally opposite elements.

(ii)   On moving down a group the size increases and thus polarizing power decreases; while going across a period from left to right the charge on the ions increases and their size decreases which lead to an increase in the polarizing power of the ions. The net result is that the element of the second period has the same polarizing power as the lower right element of the third period (i.e., diagonally placed elements). Hence their compounds are expected to have similar properties. It is interesting to not that in spite of their different valencies, the type and strength of the bonds formed; and properties of their compounds are often very similar.

 

Thus lithium resembles more with its diagonally placed element magnesium than with the other members of its own group. The important properties in which lithium resembles magnesium and thus differs from the other alkali metals are summarized below.

1. Lithium is much harder and lighter than the other alkali metals. The hardness of lithium is comparable to that of magnesium.

2. Its melting and boiling points are higher than those of the other alkali metals. It can be melted in dry air without its brilliance. The boiling point of lithium (1603 K) is comparable to that of magnesium (1373 K).

3. Due to its high ionisation energy, it is the least reactive of all alkali metals towards the various reagents, except nitrogen. Thus

(i)      lithium, unlike other alkali metals, is not affected by dry air. Magnesium is also stable in dry air.

(ii)     Unlike other alkali metals, lithium (like magnesium) decomposes water slowly liberating hydrogen.

(iii)    It reacts slowly with liquid bromine while other react violently. Magnesium also reacts slowly with liquid bromine.

(iv)    It forms a stable hydride at 973 — 1073 K while others form less stable hydrides at 673 K. Like lithium hydride, magnesium hydride, MgH₂ is fairly stable.

(v)     When burnt in oxygen, it forms only monoxide (Li₂O) while other alkali metals form higher oxides (peroxides and superoxides). Magnesium ribbon when burnt in oxygen also forms only the monoxide, MgO

(vi)    Lithium, like magnesium, does not react with acetylene gas while others react to form metal acetylides.

4. Lithium, is the only alkali metal which reacts with nitrogen and silicone to give lithium nitride. Li₃N and lithium silicide respectively. Magnesium also combines readily with nitrogen forming magnesium nitride. Mg₃N₂.

5. Lithium when heated with ammonia forms imide, Li₂NH, while other alkali metals form amides, MNH₂ .

6. Li⁺ and Mg²⁺ ions are strongly hydrated.

7. Lithium and magnesium ions show a tendency to form complexes with ammonia and water (to form hydrates) and also form a large number of organometallic compounds.

8. Lithium and magnesium oxides dissolve in water quietly while the oxides of other alkali metals dissolve in water more energetically.

9. Lithium hydroxide is much less soluble and is a much weaker base than the hydroxides of other alkali metal. Lithium hydroxide decomposes at red hot temperatures to Li₂O, while the hydroxides of other sublime, unchanged.

          2LiOH → Li₂O + H₂O

          Mg (OH)₂ → MgO + H₂O

Magnesium hydroxide is also sparingly soluble, less basic and decomposes on heating to form MgO.

10. Lithium carbonate is not stable to heat while other alkali metal carbonates are very stable towards heat. Magnesium carbonates behave like lithium carbonate.

          Li₂CO₃ →    Li₂O + CO₂

          MgCO₃  →  MgO + CO₂

11. Lithium nitrate, like magnesium nitrate, decomposes on heating giving lithium oxide and nitrogen dioxide; other alkali metal nitrates on heating give nitrites and evolve oxygen.

          4LiNO₃ →    2Li₂O + 4NO₂ + O₂

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

          2NaNO₃   2NaNO₂ + O₂

        At very high temperature NaNO₃ decomposes to NO₂ + O₂ + Na₂O.

          (2NaNO₃ → Na₂O + 2NO₂ + O₂)

12. Carbonate, phosphate, fluoride and oxalate of lithium (and also magnesium) are sparingly soluble in water while the corresponding sodium and potassium salts are highly soluble in water.

Li₂CO₃ is less soluble than Na₂CO₃ and is used as “Lithic water” for treatment of gouts.

13. Lithium compounds, particularly lithium halides, are partially covalent in nature and thus soluble in organic solvents, whereas other alkali metal halides are purely ionic and hence soluble in water.

14. Behaviour of chlorides. (i) Lithium chloride, like magnesium chloride, separates out as hydrated crystals LiCl.2H₂O like MgCl₂.6H₂O while other alkali metal chlorides do not form hydrates.

(ii)     Of all the alkali metal chlorides, only LiCl is soluble in organic solvents like alcohol and pyridine. Magnesium chloride is also soluble in alcohol.

(iii)    Lithium chloride hydrolyses in hot water though to a small extent while sodium and potassium chlorides do not hydrolyse at all. Magnesium chloride hydrolyses easily in hot water.

15. Lithium sulphate, Li₂SO₄, unlike sulphates of other alkali metals does not form alums. It is also not isomorphs with other salts.