Hydrocarbons are compound molecules consisting entirely of the elements hydrogen and carbon. Carbon atoms form linked chains to which hydrogen atoms are attached.
The simplest alkane has two carbon atoms and six hydrogen atoms (C2H6); this is the molecule ethane. The propane molecule has three carbon atoms and eight hydrogen atoms (C3H8), from which a general chemical formula can be derived; CnH2n+2.
There are three types of hydrocarbon molecules;
- Aromatic molecules which contain at least one ring in their structure.
- Saturated hydrocarbon molecules (called alkanes) in which all the atoms in the structure are linked by single covalent bonds.
- Unsaturated hydrocarbon molecules (called, alkenes, alkynes and dienes) in which some of the carbon atoms can be linked by double or even triple covalent bonds.
Hydrocarbons of various types are abundant in the rocks of the earth and are a prime source of fuel (coal, petroleum, natural gas). They are also the major component of waxes and oils and are frequently converted into important molecules such as plastic and other organic chemicals such as paint cosmetics, detergents and insecticides.
Alkanes are less dense than water and are strongly hydrophobic (they do not mix well with water or dissolve in it). A lot of energy is stored in each molecule, and this energy can readily be released by combustion (burning in oxygen) to form carbon dioxide and water.
Gasoline is a mixture of several alkanes the smallest of which is pentane (five carbon atoms) upto the largest decane (10 carbon atoms).
Larger alkanes with 17 carbon atoms, or more, are usually solids at normal temperatures.