In economic thermodynamics, an economic molecule is oft-used, metaphor-like, definition of a person in an atomic sense, as in a person viewed as a single "molecule", similar to the term human molecule. The term economics molecule tends to be used in economics or in discussing economic systems. At the level of economic molecules, a view often emerges that “life forces” are important in determining the functions and operations of an economy. 
The term economic molecule or “economic molecules” was a term used in circa 1880 by French economist Léon Walras to refer to an individual person or to people, particularly in an economic sense.  In the 1981 words of English economist Hazel Henderson: 
“[Walras] was an agrarian socialist and wanted to nationalize land, but he talked of humans as ‘economic molecules’ and gave concepts like scarcity scientific definitions analogous to heat in physics.”
The exact publication as to where Walras used this term "molécules économiques" (French) remains to be determined. In any event, the term is commonly attributed to Walras. In the 2000 words of authors Thomas Prugh, Robert Costanza, and Herman Daly: 
“Homo economicus knows neither benevolence nor malevolence, only indifference. Human beings in this model are first, last, and always extreme individuals: economic molecules, in the conception of Leon Walras, one of the founders of modern economics.”
In another section, the state:
“If it no longer makes sense—social, ecological, or political—for us to behave as the economic molecules that capitalism falsely claims is our instinctive fate, then we can be something else.”
The term economic molecules may have had influence on Walras’ student French-Italian economist Vilfredo Pareto, who in his social theories later came to speak of people as “human molecules”. 
In 1947, Dutch-born American mathematician, theoretical physicist, economist Tjalling Koopmans famous stated that time has come for us to stop denying the "human molecular hypothesis" and to begin using human molecular theory in economics:
“While it was long possible and sometimes tempting for physicists to deny the usefulness of the molecular hypothesis, we economists have the good luck of being some of the ‘molecules’ of economic life ourselves, and of having the possibility through human contacts to study the behavior of other ‘molecules’.”
Koopmans grew up during a time when many still believed that molecules did not exist. Specifically, it was not until Koopmans turned sixteen, in 1926, that French chemist Jean Perrin received the Nobel Prize in physics for proving, conclusively, the existence of molecules, by calculating Avogadro's number using three different methods, all involving liquid phase systems. First, he used a gamboge soap-like emulsion, second by doing experimental work on Brownian motion, and third by confirming Einstein’s theory of particle rotation in the liquid phase. 
Nelson | Economic agent = Human molecule
In 1992, American philosopher Alan Nelson’s used Koopmans' famous quote at the opening to his 1992 chapter “Human Molecules”, after which the idea emerged that Koopmans coined the term “human molecule” in economics, which is only partially true. To exemplify, the 2009 Oxford Handbook of Philosophical Economics defines the term human molecule as an "ontologically distinct individual" and states specifically that the term "human molecule" is “Tjalling Koopmans’ evocative coinage”.
In 1994, arguing that economics can be objective, American philosopher-economist Daniel Hausman stated: 
“We have much information immediately at our disposal about our own behavior as economic molecules, if we would only examine the grounds of our beliefs.”
Likewise, in 1996 American economics historian Thomas Rawski stated: 
“The individual choices by rice buyers and factory owners in the economy are like the movement of air molecules in a balloon … economic molecules in the modern world huddle, of course, in markets, which gives another sense in which economics is bourgeois, the townsman’s science.”
1. Bovet, Eric D. (1963). The Dynamics of Business Motivation: its Impact on the Economic Climate (pg. 39). Spartan Books.
2. Prugh, Thomas, Costanza, Robert, Daly, Herman, Goodland, Robert, Cumberland, John H., and Norgaard, Richard B. (1999). Natural Capital and Human Economic Survival, 2nd ed. (pg. 15). CRC.
3. Henderson, Hazel. (1981). The Politics of the Solar Age: Alternatives to Economics (pg. 209). New York: Anchor Press.
4. Thims, Libb. (2008). The Human Molecule (pgs. 11, 17) (preview). Morrisville, NC: LuLu.
5. Hausman, Daniel M. (1994). The Philosophy of Economics (pg. 438). Cambridge University Press.
6. Rawski, Thomas G. (1996). Economics and the Historian (pgs. 124-25). University of California Press.
7. Perrin, Jean, B. (1926). Discontinuous Structure of Matter, Nobel Lecture, December 11.
8. Kincaid, Harold and Ross, Don. (2009). Oxford Handbook of Philosophy of Economics (pg. 394). Oxford University Press.
9. Prugh, Thomas, Constanza, Robert, and Daly, Herman. (2000). The Local Politics of Global Sustainability (economic molecules, pg. 78-79, 84). Island Press.