We are looking at SiO2 and KI and comparing the boiling points and the reason for the large difference between them. KI is potassium iodide. It is an inorganic salt that is composed entirely of an ionic bond between the K+ and I- ions. Ionic bonds are very strong and stable due to their strong electrostatic interactions. In standard solid form, ionic salts like KI tend to form complex three dimensional forms called crystal lattices. These lattices minimize the space between neighboring ions, thus increasing the attractive force between them. As a result, ionic salts tend to have very high melting points because it takes an enormous amount of energy to disrupt these lattices to turn the solid into a liquid.
SiO2 is called silicon dioxide. Silicates like this comprise some of the most common minerals found on earth like sand and quartz. Like, KI, SiO2 is composed of a complex crystalline lattice structure that gives it extraordinary stability. The reason that the boiling point for SiO2 is almost double that for KI is because SiO2 forms an even more complex and stable lattice due to the fact that neighboring oxygen atoms coordinate with (donate electrons to) silicon atoms to allow four different oxygen atoms to interact with each silicon atom. As I said, this imparts an extra level of stability to the solid form of SiO2, thus increasing its boiling point in comparison with KI.