Carbon [symbol C] (from Latin: carbo “coal”) is a nonmetallic and tetravalent (rarely bivalent) chemical element—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Individual carbon atoms have an incomplete outermost electron shell. With an atomic number of 6 (six electrons and six protons), the first two electrons fill the inner shell, leaving four in the second shell. Therefore, carbon atoms can form four covalent bonds with other atoms to satisfy the octet rule. The methane molecule provides an example: it has the chemical formula CH4. Each of its four hydrogen atoms forms a single covalent bond with the carbon atom by sharing a pair of electrons. This results in a filled outermost shell.
It is solvent-insoluble, odorless and tasteless element. Its different forms (or more accurately allotropes) include one of the softest (graphite) and hardest (diamond) materials known. Other allotropic forms of carbon include amorphous carbon and fullerenes.
It also has a high affinity for chemical bonds with atoms of other low atomic weight elements (including carbon itself) and its small size makes it capable of forming multiple bonds. These properties allow for the existence of 10 million carbon compounds. Carbon compounds form the basis of all life on Earth, and the carbon-nitrogen cycle provides some of the energy produced by stars.
Carbon was already known by ancient people who produced it by burning organic material with little oxygen. In particular it was used in the production of inks. In the XVIII century Lavoisier recognized it as a simple substance (that is made of atoms of the same chemical element). Subsequently its atomic weight was determined by Berzelius.
Although it was known since ancient times, diamond was identified as an allotropic form of carbon only in 1796, thanks to the research of English chemist Smithson Tennant, who proved that diamond combustion simply produced carbon dioxide (CO2). The tetravalence of carbon was verified in 1858 by Friedrich August Kekulé von Stradonitz.
In 1961 carbon-12 was introduced as a reference by the International Union of Pure and Applied Chemistry (IUPAC) for the definition of atomic weights. Fullerene (an allotrope of carbon) was discovered as a by-product of molecular beam experiments in 1985. In the years that followed, various other forms of carbon, also part of the fullerene category, were discovered.
Isotopes of carbon
Carbon occurs naturally in three isotopes: carbon-12, carbon-13, and carbon-14. By far the most common isotope of carbon is carbon-12 (12C), which contains six neutrons in addition to its six protons. The next heaviest carbon isotope, carbon-13 (13C), has seven neutrons. Both 12C and 13C are called stable isotopes since they do not decay into other forms or elements over time. The rare carbon-14 (14C) isotope contains eight neutrons in its nucleus. Unlike 12C and 13C, this isotope is unstable, or radioactive. Over time, a 14C atom will decay into a stable product.
The vast majority of all carbon found on Earth is 12C. Almost 99% of all carbon on Earth is of this form. While only approximately 1% of all carbon on Earth is of the 13C isotopic form, 14C is still much rarer. Only one out of every trillion carbon atoms is 14C.
To gain an idea of how few 14C atoms there are compared to 12C, let’s compare one to one trillion. A trillion is a million millions. If you lined up a trillion one dollar bills, it would stretch almost from the Earth to the sun!