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Anomalous Behaviour Of Lithium

Explanation of Anomalous Behaviour of Lithium Assignment / Homework Help
As you know the physical and chemical properties and the compounds formed by alkali metals, let us see how a member of this group is related to another member of alkaline earth metals. The small size of the lithium atom and the ion make them behave differently from other elements of the same group. However, they do exhibit certain similarities. The ability to form covalent bond due to high electropositive character and the polarising power of Li+ is comparable to that of magnesium ion. This article presents information of the similarities between lithium and its diagonally located element the magnesium.

Reasons for relationship between diagonally located elements

  • As we move down the period the electropositive nature increases, but it decreases as we towards right in a period. Hence, the diagonally located elements have similar electropositive character.
  • 2. As we move down the group, the size of the atom increases and hence, the polarising power decreases. But, the size of the atom decreases as we move right in a period and hence the polarising power increases. So, the diagonally
Because of the above two reasons, the diagonally located elements lithium and magnesium show similarity in properties despite of the difference in the oxidation states.

Similarities of lithium with magnesium and dissimilarities with that of the other members of the group
  • Lithium is a very strong element and is equally strong as magnesium
  • It has high melting and boiling points. The boiling point of lithium is 1330 degree centigrade that is comparable to that of magnesium which is 1100 degrees.
  • Due to small size, the lithium atom has high ionization energy. Hence, it remains inactive to dry air, slowly reactivity with water, liquid bromine and forms a highly stable hydride unlike other alkali metals. All these properties are exactly exhibited by magnesium.
  • As magnesium it forms only monoxides. All the other alkali metal atoms form peroxides and superoxides.
  • Lithium is the only alkali metal that reacts directly with nitrogen forming nitride.
  • Both lithium and magnesium have the ability to form complex compounds with ammonia and water.
  • When the oxides of lithium and magnesium are dissolved in water, very less amount of energy is released. Other members of the alkali metals group dissolve by liberating more energy.
  • Hydroxides of lithium and magnesium are less soluble in water and are hence, considered as weak bases. The hydroxides of other members of alkali metals are stronger bases when compared to lithium hydroxide.
  • Lithium and magnesium are very less stable to heat. They form oxide and liberate carbon dioxide upon heating. Rest of the alkali metals form carbonates that are highly stable.

    Li2CO3-- => Li2O + CO2
    MgCO3-- => MgO + CO2
  • Like carbonates, nitrates of lithium and magnesium decompose readily giving rise to oxides and nitrogen dioxide. Other alkali metals give nitrites and evolve oxygen.
  • Lithium halides are covalent in nature and are hence soluble in organic solvents. The other halides are ionic in nature and are only soluble in water.
  • All alkali metal sulphates except lithium sulphate form alums.

As can be seen from the above discussion, lithium is closely related to magnesium. Similar kind of diagonal relationship is found among other elements in various groups.

Solved problems
  • Which of the following is a characteristic feature of alkali metals
    • Poor conductivity to heat and electricity
    • High oxidation potentials
    • High melting points
    • b and c
    Answer: d
  • Lithium exhibits diagonal relationship with which of the following elements
    • Mg
    • Be
    • Na
    • Cs
    Answer: a
  • Which of the following nitrates are easily decomposable
    • Lithium nitrate
    • Magnesium nitrate
    • Sodium nitrate
    • a and b
    Answer: d
  • Lithium carbonate upon heating gives
    • Carbon dioxide
    • Carbon monoxide
    • Oxygen
    • No gas
    Answer: a
  • Which of the following elements react easily with nitrogen
    • Lithium
    • Magnesium
    • Sodium
    • a and b
    Answer: d

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