Physically, they are metallic-looking, brittle (Mohs hardness 2-9), and have no structural uses. Their melting points are generally the highest among the nonmetallic elements. Valence electrons in their crystalline structures are less freely delocalized than is generally the case for metals; complete or considerable covalent bonding is present. Boron, silicon, germanium and tellurium are semiconductors; arsenic and antimony are semimetals. Their association with semiconductors, by themselves or as type III-V semiconductors (e.g. GaAs, AlSb or GaInAsSb), led to the misconception that metalloids were in-between elements. 9 When mixed with metals they can form alloys.