Sublimation

Sublimation occurs when a solid goes directly to the gas phase, without first becoming a liquid. It is most common with solid carbon dioxide (dry ice) and iodine.

At a particle level, why does this happen?

Both iodine and dry ice are non-polar molecules. Therefore, the forces between the molecules are temporary dipole-temporary dipole forces (very weak intermolecular bonds). However, their respective electron clouds are large enough to generate enough temporary dipoles to form a 3D lattice.

Image Source: https://socratic.org/questions/552d8428581e2a12d7a7324d

However, with such weak forces between these molecules, a small amount of (heat) energy is enough to break multiple intermolecular bonds. The molecules are released and moving too fast (and too far away from other molecules) to form new, temporary intermolecular bonds, required to exist in the liquid phase. Once the bonds in the solid are broken, the entire structure is compromised, so every molecule "escapes" the 3D lattice. It becomes a gas. The particles are moving too quickly and are too far apart to form the bonds required to hold them in the liquid pahse.

Iodine is the only one of these that is stable as a solid at room temperature, so we will explore that further. It is good to compare to bromine (liquid at room temperature, but readily becomes a gas as well), as they are both halogens (in Group 17).