into hydrocarbons. Crude oil—petroleum (Latin petra = rock, oleum =
oil)—is composed of a mixture of all these molecules and many more.
Petroleum refineries separate crude oil into its molecular compo-
nents, making them available to be used as fuels and other materials
in the modern industrial world. All these molecules are combustible—
that is, in the presence of oxygen they can burn and release energy as
the covalent bonds connecting the carbons and hydrogens are broken.
Fossil fuel. Complete combustion will break all of the carbon-carbon
and carbon—-hydrogen bonds and convert the hydrocarbon into a mix-
ture of carbon dioxide (CO) and water.
Carbon, as well as some other atoms, also has the capacity to partic-
ipate in bonds with other atoms in which it shares more than one elec-
tron. For example, in the ethylene molecule each carbon contributes
two (rather than one) electrons to the carbon-carbon bond, forming
what is called a double bond, drawn with a double line connecting the
atoms.
H H
£ 7 C.
H H
EthyleneEthylene is also a combustible gas, often used in welding. However,
the most widespread use of ethylene is to make polyethylene plastics
by linking many ethylene molecules together in very long chains
(polymers) of various shapes and forms.
The arrangement of carbons in a hydrocarbon molecule is not
always linear. The chains of carbons may be branched, such as in this
branched octane molecule.