Irrespective of its composition, the word molecule has been used for gaseous particles in the kinetic theory of gases. According to this theory, molecule is defined as noble gas atoms without considering its composition i.e. it is single non-bonded atom.

A molecule could be atoms of a chemical element such as oxygen O2 or of different elements such as H2O. Atoms and complexes are not regarded as single molecule because it is attached by non covalent bonds like hydrogen bonds or ionic bonds.

Molecules are part of matter and can generally spot in organic substances and biochemistry. They are also parts of oceans and atmosphere. However, crust, mantle and core of earth itself and many minerals have chemical bonds but they are not regarded as molecules. There is no particular molecule for ionic crystal, i.e. salt, and covalent crystals, i.e. network solids. However, they are made up of replicated unit cells which expanded either in plane such as grapheme or three dimensionally such as diamond or sodium chloride. The replicated structures of unit cells are also held together by metallic bonding. In glass, atoms are attached together with chemical bonds but it does not involve any repetition of unit cells which characterizes crystals.

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