Oxygen, sulphur, selenium, tellurium and polonium are members of group 16 of the periodic table. Oxygen is a gas, sulphur is a non-metallic solid, selenium and tellurium are metalloids, and polonium is a radioactive metal. Going down the group, the trend is towards an increasingly metallic character.
This trend is mirrored by the increasing electrical conductivity of the elements. Oxygen and sulphur are extremely poor conductors, selenium and tellurium are semiconductors, and polonium is the most efficient conductor. Selenium is unusual in being photoconductive: its conductivity increases as light increases.
All the members of the group react readily with most elements, although oxygen and sulphur are clearly superior in this respect. Descending from selenium to polonium, the elements show an increasing vulnerability to attack from dilute hydrochloric acid.
Selenium, tellurium and polonium have low melting and boiling points, and their compounds tend to be toxic. Polonium is a radioactive poison.
