O2 oxygen, Hydrogen H2, Iron Fe, Carbon C, are chemical elements. The molecules of a chemical element consists of atoms of the same element. 
There are 112 chemical elements. 103 of them exist in nature, while the remaining 9 had been constructed by scientists in the laboratory. The heavier of those are radioactive, that means they decay easily by themselves.
Water H2O, carbon dioxide CO2 and carbon monoxide CO, are chemical compounds. The molecules of chemical compounds consist of 2 or more atoms of different elements. Atoms are kept within the molecules of chemical compounds with electromagnetic forces, or else forces van der Waals. The stored because of these forces energy in chemical compound is called chemical energy.
There are too many chemical compounds. The compounds are divided into two broad categories. Organic chemical compounds the molecules of which containing carbon and inorganic chemical compounds the molecules of which do not contain carbon.
The molecules of a chemical compound can react with molecules of another compound or with chemical elements (e.g. oxygen), forming new chemical compounds and releasing energy in various forms but usually in the form of heat.
