Arsenic
Arsenic is found in three allotropes. Gray arsenic is the most common. It is a brittle, somewhat soft metalloid that is a little shiny. It has a density of 5.73g/cm3. It is a semiconductor. Yellow arsenic is unstable and rare. It is the most toxic allotrope. It is a soft and waxy nonmetal, similar to white phosphorus. It turns into gray arsenic in light. Its density is 1.97g/cm3, much lighter than gray arsenic. It is created when arsenic vapor (made by heating arsenic very hot) is cooled very fast by something like liquid nitrogen. Black arsenic is similar to red phosphorus. It is a brittle, black, and shiny nonmetal. It does not conduct electricity. Since gray arsenic is the most common, it is usually referred to as arsenic.