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Carbon (pronounced
/ˈkɑrbən/) is the
chemical element with
symbol C and
atomic number 6. As a member of
group 14 on the
periodic table, it is
nonmetallic and
tetravalent—making four electrons available to form
covalent chemical bonds. There are three naturally occurring
isotopes, with
C and
C being stable, while
C is
radioactive, decaying with a
half-life of about 5730 years. Carbon is one of the
few elements known since antiquity.The name "carbon" comes from
Latin language carbo,
coal, and, in some
Romance and
Slavic languages, the word carbon can refer both to the element and to coal.
There are several
allotropes of carbon of which the best known are
graphite,
diamond, and
amorphous carbon. The
physical properties of carbon vary widely with the allotropic form. For example, diamond is highly
transparent, while graphite is
opaque and black. Diamond is among the hardest materials known, while graphite is soft enough to form a streak on paper (hence its name, from the Greek word "to write"). Diamond has a very low
electrical conductivity, while graphite is a very good
conductor. Under normal conditions, diamond has the highest
thermal conductivity of
all known materials. All the allotropic forms are solids under normal conditions but graphite is the most
thermodynamically stable.
All forms of carbon are highly stable, requiring high temperature to react even with oxygen. The most common
oxidation state of carbon in
inorganic compounds is +4, while +2 is found in
carbon monoxide and other
transition metal carbonyl complexes. The largest sources of inorganic carbon are
limestones,
dolomites and
carbon dioxide, but significant quantities occur in organic deposits of
coal,
peat,
oil and
methane clathrates. Carbon forms more
compounds than any other element, with almost ten million pure
organic compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.
Carbon is one of the
least abundant elements in the Earth's crust, but the
fourth most abundant element in the universe by mass after
hydrogen,
helium, and
oxygen. It is present in all known
lifeforms, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen.This abundance, together with the unique diversity of
organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on
Earth, make this element the chemical basis of all known
life.
