A molecule is an electrically neutral group of several atoms held together as a result of covalent chemical bonds. Substances are distinguished from ions just by their electrical charge. However, in quantum physics, all natural chemistry, and biochemistry, the term molecule is often utilized less strictly, also being used on polyatomic ions.
In this kinetic theory of gases, the term molecule is usually used for any gaseous particle irrespective of its composition. According to this classification noble gas atoms are considered molecules even though they are composed on the single non-bonded atom. 
A molecule may involve atoms of a single chemical element, as with oxygen (O2), or even of different elements, as with water (H2O). Atoms and complexes connected by non-covalent bonds including hydrogen bonds or ionic bonds are generally not considered single molecules. 
Molecules as different parts of matter are common in organic substances (and for that reason biochemistry). They also make up most of the oceans and atmosphere. Nevertheless, the majority of familiar solid substances that is known, including most of the minerals that define the crust, mantle, and core of the Earth, contain many chemical type bonds, but are not made of identifiable molecules. Also, no typical molecule may be defined for ionic crystals (salts) and covalent crystals (network solids), although these are often composed of duplicated unit cells that extend either in the plane (such as in graphene) or three-dimensionally (such as in diamond, quartz, and sodium chloride). The theme of recurrent unit-cellular-structure also holds for some condensed phases with metallic bonding, which means that solid metals are also not made of substances. In glasses (solids which exist in a vitreous disordered state), atoms may additionally be held together by chemical bonds without occurrence of any definable molecule, but also without many of the regularity of repeating devices that characterises crystals.transformers