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).
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?