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Question

Which of the following compounds has the lowest melting point ?


A
CaCl2
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B
CaBr2
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C
CaI2
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D
CaF2
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Solution

The correct option is C $$CaI_2$$
For covalent halides, the melting and boiling point are expected to follow the trend,
Flouride<chloride<bromide<iodide
This trend results from the increase in the magnitude of Vander Waal's forces holding individual molecules together as number of electrons in the halogen increases.
If the halides compared are ionic compounds, they can be expected to be solids at room temperature with melting point that follows opposite trend, flouride>chloride>iodide, since decreasing lattice energies are present for the larger halide ions.
Within a group, or within a set of halides of a given element, the contrating trends in melting point among ionic-covalent materials can produce some reverse
For eg:
          Melting Points
$$Ca{ F }_{ 2 }$$    $$1360℃$$
$$Ca{ Br }_{ 2 }$$  $$730℃$$--
$$Ca{ Cl }_{ 2 }$$  $$772℃$$
$$Ca{ l }_{ 2 }$$    $$740℃$$
The earlier part of trend is consistent with the decreasing lattice energies in ionic compounds, while later part is consistent with Van der Waal's forces as halogen atoms in the covalent lattice becomes large.

Chemistry

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