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The table below shows the melting points, in degrees ºC, for the binary halides of five alkali metals. Notice the trend for each anion as you go from lithium to sodium, potassium, rubidium, and cesium.

$Li$ $Na$ $K$ $Rb$ $Cs$
$F$ 845 993 858 795 682
$Cl$ 605 801 770 718 645
$Br$ 550 747 734 693 636
$I$ 449 661 681 647 626

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In general, the melting point increases from the lithium salt to the sodium salt, and then decreases as one continues across to potassium, rubidium, and cesium.

Which of the following offers a reasonable explanation for the unique behavior of lithium, compared to the other four members of the family?


Lithium has the highest electronegativity of all the alkali metals.


The lithium ion is so small that it is able to effectively polarize the electron clouds of the adjacent halide ions. As a result, the $Li-X$ bond has a significant amount of covalent character. $X$ represents the halide ion.


Atoms in the second period of the table, including lithium, have only s and p sublevels in their valence shells.


Because its lone valence electron is so close to the nucleus, it takes a great deal of energy for lithium to form a cation.

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