Many properties of covalent molecular substances are exemplified by hydrocarbons, compounds that contain only the elements carbon and hydrogen. Carbon is unique among the elements in that it can form long chains of stable carbon-carbon bonds. And because a carbon atom has four valence electrons, four covalent bonds can form per carbon atom. When all other bonds in a chain of carbon atoms are to hydrogen atoms, the molecule is a hydrocarbon.
In addition to long chains, hydrocarbon molecules can have chains with branches, chains folded back on themselves to form rings, and any of those can also include double or triple bonds. Many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have only been prepared in the laboratory.
One of the most important reaction of hydrocarbons is combustion. The simplest example is combustion of methane:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔrH = −802.3 kJ/mol
Combustion of hydrocarbons is highly exothermic, so hydrocarbons are excellent fuels. For example, methane (CH4) is the principal component of natural gas, LP gas is mainly propane (C3H8) and butane (C4H10). Other hydrocarbon fuels include acetylene, diesel fuel, jet fuel, and heating oil. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons.