A model of a methane molecule and two oxygen molecules, the reactants  when we burn methane (natural gas) in air.  The methane molecules is made of one carbon atom (black)  and four hydrogen atoms (white).   Methane is  a  hydrocarbo.  It is also a fossil fuel.  The element oxygen occurs in the atmosphere as two oxygen atoms (red) combined into a molecule.   When the methane burns, the above reactants become rearranged into the products below, with the release of energy.

 These are the products formed, carbon dioxide and water,  when methane burns in oxygen.  The leftmost molecule is carbon dioxide, made up of one carbon atom (black)  and two oxygen atoms (red).  Each of the two water molecules is made up of two hydrogen atoms and one oxygen atom.  Carbon dioxide is the greenhouse gas which is put into the atmosphere when methane, or any fossil fuel is burned.  This is  the source of our global warming.