Question

Why is fluorine a gas, bromine a liquid, and iodine a solid, at room temperature?

Answer 1

Hint: Nonmetals in group 17 (or VII) of the periodic table are halogens. The size of the atoms gets bigger as you go along the group. Fluorine has the weakest link of any diatomic molecule due to repulsion between the electrons of the tiny atoms. The halogens are a set of five chemically related elements in the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (A) (At). Tennessine (Ts), an artificially produced element, may also be a halogen. This group is designated as group 17 in contemporary IUPAC nomenclature.

Complete answer:
The boiling temperatures of halogens rise as the strength of Van der Waals forces down the group increases. As a result, the physical state of the elements decreases from gaseous fluorine to solid iodine as they progress along the group.
The reason for this is that 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. Fluorine and chlorine are gases exclusively at temperatures between -7 and 59 degrees Celsius, while bromine and iodine are solids. Despite the fact that intermolecular interactions are weak individually, they are very strong collectively, and the many forces between iodine molecules implies that more heat energy is required to break these intermolecular bonds and separate the molecules, making iodine a solid.
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.

Note:
Halogens are used in the chemical, water and sanitation, plastics, pharmaceutical, pulp and paper, textile, military and oil industries. Bromine, chlorine, fluorine and iodine are chemical intermediates, bleaching agents and disinfectants.