The halogen that goes directly from the solid phase to a gas is iodine (I₂). This process is called sublimation.

Here's why:

* Iodine's low melting point: Iodine has a relatively low melting point (113.7 °C) and a low boiling point (184.3 °C). This means that at standard atmospheric pressure, it can easily transition from a solid directly to a gas when heated, skipping the liquid phase.

* Sublimation: This process is common for substances with weak intermolecular forces and relatively high vapor pressures. Iodine's structure allows for easy sublimation.

While the other halogens (fluorine, chlorine, bromine) can also sublime under specific conditions, iodine is the most readily observable example at standard conditions.