Atomic force microscopy (AFM) image of a PTCDA molecule, in which the five six-carbon rings are visible. 
A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. 
AFM image of 1,5,9-trioxo-13-azatriangulene and its chemical structure. 
molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion.     In  quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions.
A molecule may be
homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O 2); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H 2O). In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the noble gases are individual atoms. Atoms and complexes connected by  non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules.
Concepts similar to molecules have been discussed since ancient times, but modern investigation into the nature of molecules and their bonds began in the 17th century. Refined over time by scientists such as
Robert Boyle, Amedeo Avogadro, Jean Perrin, and Linus Pauling, the study of molecules is today known as molecular physics or molecular chemistry.
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"Molecule". . 22 January 2016. Encyclopædia Britannica Archived from the original on 3 May 2020 . Retrieved . 23 February 2016