Their properties go from nonmetallic in oxygen and sulfur to metalloid in selenium and tellurium to metallic in polonium. They are strongly nonmetallic. The halogens include fluorine, chlorine, bromine, iodine, and astatine. They are the most active nonmetals. The chemical reactivity of the halogens is due to the number of electrons in the outermost energy levels of their atoms.
Fluorine is the most active halogen.
They have low melting points and boiling points. In the gas phase they exist as diatomic elements. Halongens combine readily with metals to form a class of compounds known as salts. Noble Gasses are colorless gasses that are extremely unreactive. Because do not readily combine with other elements to form compounds, the noble gasses are called inert.