Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be a metal if it could form a condensed phase. Astatine may be metallic in the liquid state on the basis that elements with an enthalpy of vaporization (∆H vap) greater than ~42 kJ/mol are metallic when liquid. Such elements include boron, silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆H vap of diatomic astatine are 50 kJ/mol or higher; diatomic iodine, with a ∆H vap of 41.71, falls just short of the threshold figure.