Molecule

Molecule is a noun which, although not coined till the growth in scientific investigation of the seventeenth century, did not have a firm definition until nineteenth century developments in chemistry, and has since taken on a more precise technical sense, as well as some rather different figurative meanings.

• Its basic use nowadays is in modern Chemistry, where it means a group of atoms (at least two) bonded together to form the smallest unit of a substance that can take part in chemical reactions independently, "a group of atoms mechanically indivisible" (Roscoe, H. E. (1866) Lessons in Elementary Chemistry, (1st edition) London, Macmillan, xvi. 169, cited OED). (Some chemical elements only exist naturally as molecules; their atoms do not 'float free'. They can be recognized in their appearance in formulae etc with the subscript 2, as normal oxygen and hydrogen are O₂ and H₂.) AWE can't say any more; readers who want to know more must research their technical subjects.
• Originally, molecule was defined as "one of the minute discrete particles of which material substances were thought to be composed" (OED), 'the smallest possible 'bit' of any substance'. (This is approaching the modern understanding of 'atom'.) In general, this has attained some figurative use as 'the smallest possible unit [of the system being discussed]', such as an individual voter; or, more colloquially, by an exasperated parent to a young child: "You haven't a molecule of sense."
• It became used in the nineteenth century for two obsolete meanings:
  • in Chemistry for what is now defined as a mole in the sense of a a unit of measurement in the SI (Mol).
  • In Biology, for a tiny fragment of a structure, especially a nerve, that is practically invisible even under an optical microscope.
• It now has a figurative application in Psychology and Social Sciences to mean the behaviour of an individual, or a small sub-section of a group, as opposed to the behaviour of the larger group, which may be described as molar. It may be illustrated by the behaviour of one fish inside a shoal.

The adjective molecular is applied in this last sense, as well ass the principal use in modern Chemistry, and frequently to label that branch of a larger subject which deals with matter at a sub-microscopic level, as Molecular Biology, for examplke, deals with living beings at the chemical level, "the branch of science concerned with the formation, organization, and activity of macromolecules essential to life (nucleic acids, proteins, etc.)" (OED). Molecular Electronics deals with how current may

Etymological note: molecule is derived from the noun 'a mole' in the sense of 'a mass', + the suffix -cule, Latin -cula, meaning '[very] small'. Its earliest appearance in print in England was in the Latin form molecula, in Gassendi P. (c. 1665) Syntagmatis Philosophici (cited OED), where it is defined in the phrase "a little masse, or molecula".