Most metalloids, like the less electronegative nonmetals, form allotropes. Some authors count metalloids as nonmetals with weakly nonmetallic properties. Chemically the metalloids generally behave like (weak) nonmetals. In between metals and nonmetals are metalloids, which have intermediate or mixed properties. Astatine forms covalent compounds with nonmetals, including hydrogen astatide HAt and carbon tetraastatide CAt. Covalent bonds are generally formed between two nonmetals. Some solid nonmetals (sulfur, phosphorus) react vigorously in liquid air temperature fluorine. Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals. The reactive nonmetals vary in their nonmetallic character. Metals are ductile while nonmetals are not. He classified these substances into metals and nonmetals. Metalloids usually look like metals but behave largely like nonmetals. Period 3 and heavier nonmetals can form hypervalent fluorides. Chlorine is slightly more selective, but still reacts with most metals and heavier nonmetals. Antimony has fewer tendencies to anionic behaviour than ordinary nonmetals.