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Consider the following table showing the melting and boiling points of three of the alkali metals, along with their specific gravities.

Metal MP (ºC) BP (ºC) Sp. Gr.
$Na$ 97.8 883 0.97
$K$ 63.7 759 0.86
$Rb$ 39.5 688 1.53

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Which of the following statements offers the best explanation of the data in the table concerning the observed trends, going from sodium to potassium to rubidium?


As the metal atoms increase in size, they get farther apart from each other, so their melting and boiling points decrease. The strength of dispersion forces among neutral atoms decreases with increasing distance between the atoms.


The valence electrons get farther and farther from the nucleus as the atomic number increases, weakening the pull of the nucleus on the valence level.


The valence electrons in the metallic lattice are farther from the nuclei, so they do not bind the atoms as tightly to each other. As a result, less thermal energy is needed to disrupt the solid lattice or to overcome the attractive forces between liquid atoms.


The alkali metals become more dense with increasing atomic number.

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