question_answer 44)

  The order of decreasing ionisation enthalpy in alkali metals is : (a)\[~Na\text{ }>\text{ }Li\text{ }>\text{ }K\text{ }>\text{ }Rb\]                                                (b) \[Rb\text{ }<\text{ }Na\text{ }<\text{ }K\text{ }<\text{ }Li\]              (c)\[Li>Na>K>Rb~~~~\]                                                                (d) \[K<\text{ }Li\text{ }<\text{ }Na\text{ }<\text{ }Rb\]

Answer:

  (c) The atoms of alkali metals have the largest size in their respective periods and thus, valence electron can be easily removed. As the size of the atoms increases on moving down the group, the ionisation enthalpy decreases accordingly.