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Hi Gill,

Fossil fuels like oil, gas and coal contain carbon bound up with other elements in molecules. (Coal itself is made up of molecules of pure carbon but there are usually impurities embedded in it). When the fuel is burned the molecules are broken apart into smaller molecules or individual atoms. This is what releases the energy. The individual atoms of each element are then free to join with other atoms to form new molecules. Carbon combines very readily with oxygen to form carbon monoxide (1 carbon atom + 1 oxygen atom) and carbon dioxide (1 carbon atom and 2 oxygen atoms).

Hope that helps!

Dave P.