At room temperature, chlorine is a gas while iodine is a solid. This is because of the difference in their intermolecular forces. Chlorine molecules are non-polar and have weak van der Waals forces between them. Iodine molecules, on the other hand, are polar and have stronger van der Waals forces as well as dipole-dipole interactions between them. These stronger intermolecular forces hold the iodine molecules more closely together, making it a solid at room temperature.

In contrast, the weaker van der Waals forces between chlorine molecules allow them to move more freely and exist as a gas at room temperature.