Sulfur, iodine, and magnesium are all nonmetals. Nonmetals are elements that lack the properties of metals, such as high electrical and thermal conductivity, malleability, ductility, and a shiny appearance. Nonmetals are generally poor conductors of heat and electricity, and they are brittle and non-malleable. Sulfur, iodine, and magnesium are all relatively reactive elements, meaning that they readily undergo chemical reactions with other elements.