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Consider the table below showing the melting points of the halides of four alkali metals. The numbers in the boxes are the melting temperatures of the corresponding alkali metal halides, all in degrees Celsius (ºC).

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

$\ $
For each of the alkali metals, there is a general decrease in melting point with increasing atomic number of the halogen with which it is bonding. There is a steady decrease in melting point as one reads down each column of the table.

Which of the following is the BEST explanation for this pattern?


As the atomic number of the halogen increases, so does its ionic radius. As the distance between ion centers increases, the electrostatic force of attraction decreases.


As the atomic number of the halogen increases, so does its effective nuclear charge. The electrostatic force of attraction between the ions increases with increasing electrical charge.


As the atomic number of the alkali metals increases, their effective nuclear charge decreases, weakening the electrostatic forces of attraction among the ions.


As the halide ions increase in atomic number, they also increase in size. The larger anions repel one another more effectively, weakening the strength of the ionic crystal.

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