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Oxygen
An element with atomic symbol O, atomic number 8, and atomic weight [15.99903; 15.99977]. It is the most abundant element on earth and essential for respiration.
Year introduced: 1963
Chalcogens
The elements OXYGEN; POLONIUM; SELENIUM; SULFUR; and TELLURIUM; that form group 16 (formerly group VI) of the periodic table.
Year introduced: 1998
Inorganic Chemicals
A broad class of substances encompassing all those that do not include carbon and its derivatives as their principal elements. However, carbides, carbonates, cyanides, cyanates, and carbon disulfide are included in this class.
Gases
The vapor state of matter; nonelastic fluids in which the molecules are in free movement and their mean positions far apart. Gases tend to expand indefinitely, to diffuse and mix readily with other gases, to have definite relations of volume, temperature, and pressure, and to condense or liquefy at low temperatures or under sufficient pressure. (Grant and Hackh's Chemical Dictionary, 5th ed)
Elements
Substances that comprise all matter. Each element is made up of atoms that are identical in number of electrons and protons and in nuclear charge but may differ in mass or number of neutrons.