Among halogens, iodine is solid at room temperature while Br2 is liquid and Cl2 and F2 are gases at room temperature, why?

Explanation

As you move down the Group, the attraction forces between the molecules of these elements get stronger. Each substance's molecules are attracted to one another by dispersion (London) intermolecular forces. The balance between the kinetic energy of the molecules and their intermolecular attractions determines whether a material is a solid, liquid, or gas. The electrons in fluorine are firmly bound to the nuclei. The London dispersion forces are generally modest because electrons have limited opportunity to stray to one side of the molecule. The electrons are more away from the nuclei as we progress from fluorine to iodine, allowing the electron clouds to deform more easily. The dispersion forces in London grow increasingly powerful. The molecules will all be solids at a low enough temperature. They'll all be gases at a high enough temperature. Now Because the fluorine molecule has a low molecular weight and weak intermolecular interactions, it exists as a gas at ambient temperature. Bromine, on the other hand, has a slightly higher molecular weight than fluorine and has stronger intermolecular interactions, thus it persists as a liquid at ambient temperature. Because iodine has a large molecular weight and strong Van Der Waals forces, it exists as a solid at normal temperature.