Oxygen and its compounds play a key role in many of the important processes of
life and industry. Oxygen in the biosphere is essential in the processes of
respiration and metabolism, the means by which animals derive the energy needed
to sustain life. Furthermore, oxygen is the most abundant element at the surface
of the Earth. In combined form it is found in ores, earths, rocks, and gemstones,
as well as in all living organisms. Oxygen is a gaseous chemical element in
Group VA of the periodic table. The chemical symbol for atomic oxygen is O, its
atomic number is 8, and its atomic weight is 15.9994. Elemental oxygen is known
principally in the gaseous form as the diatomic molecule, which makes up 20.95%
of the volume of dry air. Diatomic oxygen is colorless, odorless, and tasteless.
Two 18th-century scientists share the credit for first isolating elemental
oxygen: Joseph PRIESTLEY (1733-1804), an English clergyman who was employed as a
literary companion to Lord Shelburne at the time of his most significant
experimental work, and Carl Wilhelm SCHEELE (1742-86), a Swedish pharmacist and
chemist. It is generally believed that Scheele was the first to isolate oxygen,
but that Priestley, who independently achieved the isolation of oxygen somewhat
later, was the first to publicly announce his findings. The interpretation of
the findings of Priestley and the resultant clarification of the nature of
oxygen as an element was accomplished by the French scientist Antoine-Laurent
LAVOISIER (1743-94). Lavoisier\'s experimental work, which extended and improved
upon Priestley\'s experiments, was principally responsible for the understanding
of COMBUSTION and the establishment of the law of conservation of matter.
Lavoisier gave oxygen its name, which is derived from two Greek words that mean
"acid former." Lavoisier held the mistaken idea that oxides, when dissolved in
water, would form only acids. It is true that some oxides when dissolved in
water do form acids; for example, sulfur dioxide forms sulfurous acid. Some
oxides, however, such as sodium oxide, dissolve in water to form bases, as in
the reaction to form sodium hydroxide; therefore oxygen was actually
inappropriately named.


Oxygen is formed by a number of nuclear processes that are believed to occur in
stellar interiors. The most abundant isotope of oxygen, with mass 16, is thought
to be formed in hydrogen-burning stars by the capture of a proton by the
isotopes of nitrogen and fluorine, with the subsequent emission of, respectively,
a gamma ray and an alpha particle. In helium-burning stars the isotope of carbon
with mass 12 is thought to capture an alpha particle to form the isotope with
mass 16 with the emission of a gamma ray. In the terrestrial environment oxygen
accounts for about half of the mass of the Earth\'s crust, 89% of the mass of the
oceans, and 23% of the mass (and 21% of the volume) of the atmosphere. Most of
the Earth\'s rocks and soils are principally silicates. The silicates are an
amazingly complex group of materials that typically consist of greater than 50
(atomic) percent oxygen in combination with silicon and one or more metallic
elements. Several important ores are principally oxides of the desired metals,
such as the important iron-bearing minerals hematite, magnetite, and limonite
and the most important aluminum-bearing mineral, BAUXITE (a mixture of hydrated
aluminum oxides and iron oxide).


Three naturally occurring isotopes of oxygen have been found: one with mass 16
(99. 759% of all oxygen), one with mass 17 (0.037%); and one with mass 18
(0.204%). The rarer isotopes, principally the latter, find their major use in
labeling experiments used by scientists to follow the steps of chemical
reactions. If oxygen at a pressure of one atmosphere is cooled, it will liquefy
at 90.18 K (-182.97 deg C; -297.35 deg F), the normal boiling point of oxygen,
and it will solidify at 54.39 K (-218.76 deg C; -361.77 deg F), the normal
melting point of oxygen. The liquid and solid forms of oxygen have a pale blue
color. Several different structures are known for solid oxygen: solid type III,
from the lowest temperatures achievable to 23.66 K; type II, from 23.66 to 43.76
K; and type I, from 43.76 to 54.39 K. The critical temperature for oxygen, the
temperature above which it is impossible to liquefy the gas no matter how much
pressure is applied, is 154.3 K (-118.9 deg C; -181.9 deg F). The pressure of
liquid and gaseous oxygen coexisting in equilibrium at the critical temperature
is 49.7 atmospheres. Oxygen gas exhibits a slight but important solubility in
water. Molecular oxygen dissolved in water is required by aquatic organisms for
their metabolic processes and