A metalloid is a chemical element with properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid, nor is there complete agreement as to which elements are appropriately classified as such.oron, silicon, germanium, arsenic, antimony, and tellurium are recognised commonly as metalloids. Depending on the author, one or more from selenium, polonium, or astatine sometimes are added to the list. Boron sometimes is excluded, by itself, or with silicon. Sometimes tellurium is not regarded as a metalloid.