Atoms share electrons so that they have complete octets. Chemical bonds vibrate. The molecular mass of a substance is the mass which contains a mole of molecules of that substance. The mole concept provides an easy bridge between the number of molecules and the mass of a substance. Chemical calculations therefore routinely use the mole concept.
Natural petroleum is a complex mixture cotainins thousands of compounds.
It is refined to yield a single compound or a mixture of compounds having certain desired properties. Other compounds are then added to give the gasoline other desirable properties, such as high octane number. The have the same number of electrons in their outer shells. They have similar chemical properties. They have similar physical properties.