Question

chemistry-At 1.0 atm and 25.0C Cl2 exists as a gas and I2 exists as a solid

Answer 1

One way of explaining this is by looking at the intermolecular force of attraction (IMFA) present in these halogens. First of all, you should be aware that these elements exist as diatomic molecules. Looking at their IMFA, neither one of them exhibits H-bonding since all of them are nonpolar (cancelled dipole moments). Thus, we'll just be looking at what we are calling the London Dispersion Force (LDF) at these elements. If you still remember you Chemistry, as the molecular mass (MM) increases, LDF also increases. A stronger LDF means that interaction between the particles (or in this case, elements) gets stronger, therefore, their molecules are supposed to be "more attached" to each other. Looking at the molecular masses. Chlorine (Cl2) has the lowest MM thus weakest LDF while Iodine (I2) has the highest MM thus strongest LDF. We can further relate this to their physical states, solid has the greatest molecular interactions while gas has the weakest molecular attractions. That's why we can observed these elements at different physical states at room temperature. To summarize, it's just the difference in their IMFA, particularly LDF. :)