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Compounds Formed By Elements Of Group IA

Compounds Formed by Elements of Group IA Assignment / Homework Help
Elements of group IA are highly reactive and form a variety of compounds. Following is a list of compounds formed by alkali metals.

  • Oxides:

    Alkali metals form oxides when they are burnt in air. The nature of the oxides varies among the elements. For example lithium forms monoxide (Li2O), sodium forms peroxide (Na2O2) and the remaining elements mainly form superoxides with very little amounts of peroxides. These reactions are given as below:

    4Li + O2 ---- => 2 Li2O (Lithium monoxide)
    Na + O2 ---- => Na2O2 (Sodium peroxide)
    K + O2 ---- => K O2 (Potassium superoxide)

    The reason for formation of peroxides and superoxides as we move down the group is due to increased stability of the larger cations through lattice energy.

    Lithium on the other hand, due to stronger attraction exerted by the nucleus due to smaller size attracts the electrons strongly preventing the reaction with another oxygen atom. Hence, it forms only monoxide but not peroxide or superoxides.

    Monoxides formed by alkali metals are highly soluble in water and form hydroxides giving strongly alkaline solution.
  • Hydroxides:

    As said above hydroxides are formed when the metal oxides are dissolved in water. These hydroxides dissolve very easily in water and in alcohol. They are the highly alkaline in nature and hence, serve as strong bases. The basic character of the hydroxides increases as we move down the group. This is mainly due to increase in the internuclear distance which increases the ease of separation of the hydroxide ion from the cation.

    Another interesting feature of these hydroxides is the stability to heat. Even when they are boiled red hot they do not lose their water content. But they start emerging vapours at about 400 degrees.
  • Halides:

    Alkali metal halides can be formed very easily by the combination of the metal with a halogen. Alkali metal halides are represented as M+X-. Caesium and Rubidium because of their large size form polyhalides of the type MX3.

    All metal halides are easily soluble in water, good conductors of electricity in fused state and have high melting and boiling. Due to high lattice energy alkali metal halides are highly stable.
  • Carbonates and bicarbonates:

    Alkali metals react with carbon dioxide and form carbonates of the form M2CO3. Due to the high electropositive character of the alkali metals, carbonates and bicarbonates are highly stable. This stability of carbonates increases as we move down the group. However, lithium carbonate is weak and easily decomposes to heat. This is due to stronger attraction of lithium ion over the oxygen atom that is associated with carbon dioxide. As a result of this a stronger Li--O bond is formed. The oxygen atom is very weakly attached to the carbon and gets easily decomposed upon heating. Alkali metals are the only compounds that form highly stable bicarbonates. As said before, this is mainly due to their strong electropositive character.
  • Hydrides:

    Alkali metals form hydrides of the type MH. These are very easily soluble in water and are very less stable. Alkali metal hydrides form hydroxides when dissolved in water and liberate the hydrogen. Hence, the hydrides serve as very strong reducing agents. For example, lithium aluminium hydride is a very strong reducing agent and is used in many reactions.
  • Nitrides:

    Alkali metals react with nitrogen to form nitrides. However, only lithium reacts directly with nitrogen to form nitrides.

    6Li + N2 -- => 2Li2N

    Sodium and potassium nitrides are formed only after heating the metallic acid with strong liquid ammonia.

Solved problems
  • Which of these elements form nitrides immediately?
    • Na
    • K
    • Rb
    • Li
    Answer: d
  • Which of the following behave as strong reducing agents
    • Alkali metal oxides
    • Alkali metal hydrides
    • Both
    • None
    Answer: b
  • An example for strong reducing agent
    • LiAlH4
    • NaH
    • Potassium hydride
    • None
    Answer: a
  • The low stability of carbonates is due to
    • High electropositive nature of alkali metals
    • High electronegative nature of alkali metals
    • Both
    • None
    Answer: a
  • Superoxide is formed by which of the following elements
    • Li
    • Na
    • K
    • Rb
    Answer: Rb

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