Question

Which element of group $17$ undergoes sublimation?

Answer 1

We know that, at room temperature chlorine and fluorine are gases, and bromine and iodine are solid. And sublimation is a process where a substance goes directly from the solid to the gas state, without passing through the liquid state.

Complete answer:
Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid. The reverse process of sublimation is deposition or de-sublimation, in which a substance passes directly from a gas to a solid phase.
Group $17$ elements are collectively called halogens, and they are reactive nonmetals including fluorine, chlorine, bromine, and iodine.
From halogens only Iodine undergoes sublimation and in essence directly changes into a gaseous state without melting.
For a halogen to undergo this type of phase transition, it needs to be solid. Iodine is solid at room temperature. In the laboratory, solid iodine is kept in sublimation apparatus and is heated under vacuum. Because of the reduced pressure, the non-volatile impurities remain behind.

Note:
Remember it is important for the element to be in a solid state while undergoing sublimation. And iodine is solid at room temperature. Also, iodine has many commercial uses. Iodide salts are used in pharmaceuticals and disinfectants, printing inks and dyes, catalysts, animal feed supplements and photographic chemicals.