Question

What type of crystals does iodine, dry ice (CO2) , ice and plastic form?

The answer is molecular, but i dont know why..anyone explain thanks

Answer 1

Your really important clue is that all four are made of nonmetal elements (those on the right-hand side of the Periodic Table). That generally means any compounds are made up of molecules -- groups of atoms glued together with chemical bonds. When a compound freezes, it usually does so as whole molecules. That is, the molecules do not break up or lose their identity when the solid crystal is formed. If you look at a crystal of dry ice under an exceedingly powerful microscopic (e.g. an STM), you would see individual molecules of carbon dioxide in it.

This is by contrast with an ionic compound, such as NaCl, in which the individual ions (Na+ and Cl-) form a cystal and there is no identifiable "NaCl" molecule, and also with metals or alloys of metals, which also do not form molecules when they form solids.

In the case of CO2, ice and plastic, you can be confident you have a compound made of molecules, because none of these is an element itself. In the case of iodine, it's a little trickier, because iodine is an element. You just have to know that iodine, the element, exists as I2 molecules under standard conditions. (There are 7 elements in the Periodic Table that exist as diatomic molecules in their standard state: hydrogen plus the 7 elements forming a large number "7" on the right-hand side of the table: N, O, F, Cl, Br and I.) Since iodine exists as molecules, it will stay in the form of molecules when it freezes.

The case of ice is slightly tricky in that there is strong hydrogen bonding between the water molecules, and the individual hydrogen atoms do not necessarily stay connected to the same oxygen. Some authors do not classify ice as a molecular solid, but rather as what's called a "network solid," which carbon (in its diamond form) and silicon dioxide also form. It's to some extent a matter of taste and what properties of the solid you want to emphasize.

Answer 2

Thanks for your reply :) I understand it now