The semi-metals, also known as metalloids, are Boron (atomic number 5), Silicon (14), Germanium (32), Arsenic (33), Antimony (51), Tellurium (52) and Polonium (84). They are so named because they share properties of both metals and nonmetals.
As such, they tend to be lustrous (like metals), brittle (as opposed to the malleability and ductility of metals) semiconductors with other properties intermediate between metals and nonmetals.
Is Astatine a halogen or metalloid?
Depending on whom you ask, astatine is either a halogen or a semimetal. The confusion stems from the short half-life of astatine, the longest-lived isotope having a half-life of only 8.1 hours. It is so radioactive that scientists cannot keep a sample around long enough to assess whether it does indeed have the properties of a metalloid. Rather, the heat generated by its decay melts and vaporizes whatever amount of sample remains.