|Left: Basic family: mother, father, and daughter (in a pool). Right: Bound state representation of a family: MxFyBc (father Mx, mother Fy, child Bc), connected by human chemical bonds (photon bonds γ). |
In human chemistry, the first to refer to a a family as a type of combined "chemical", via human chemical theory or human molecular theory, was Thomas Dreier:
“Every family is a combination of human chemicals.”— Thomas Dreier (1948), We Human Chemicals
Dreier elaborated further on this basis in his chapter on the human chemistry of family life. 
In the 2001 book Leaving and Clinging, American writer Paul Peachey used the concept of the “family molecule”, being the attachment of two or more human molecules, to argue that the family is the core molecule of society. Peachey seems to cull many of his ideas on human bonding and human molecules from American sociologist Robert Nisbet, whom he quotes often, for example: 
“It should be obvious that family, not the individual, is the real molecule of society, the key link of the social chain of being. The family [is] the societal germ or ‘molecule’ that Nisbet proclaims.”
In another instance, he states: “The family is the social molecule, it is not the first instance because it produces and socializes the young, but because it is the cradle of covenanting freedom. The often life-long trauma experienced by children of divorces testifies to the germinal (or molecular) significance of pair-bonding in the creation of the human.” Most of his reasoning seems, however, to be used in loose metaphor. Below is one example:
“To put the matter metaphorically, only when the family molecule begins to dissolve into individual atoms can the rational reconstruction or resynthesis that we call modernization be undertaken.”
In 2007, American electrochemical engineer Libb Thims began to describe families, chemically, using three points of view: as “bound states” of human molecules; as di-humanide or tri-humanide molecules, etc., depending on geometry and molecule count; or using human molecular orbital theory as overlaps of probability orbitals. 
In the 1940s and 1950s, supposedly, concepts from systems theory, cybernetics, and psychodynamics began to filter over into family therapy books, carrying over concepts of boundaries, entropy, negentropy, openness and closedness, etc., to develop theories on family issues. 
In 2007, Herbert and Irene Goldenberg, in their Family Therapy, state: 
“Families vary in the extent to which they are open systems; relatively closed systems run the risk of entropy or decay and disorganization.”
The term ‘relatively closed system’, however, is Carl Jung psychodynamics term, not being proper thermodynamics.
● Family molecule
● Nuclear family
1. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (pg. 183-86). (preview), (Google books). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (Google books). Morrisville, NC: LuLu.
2. Dreier, Thomas. (1948). We Human Chemicals: the Knack of Getting Along with Everybody (ch. “The Human Chemistry of Family Life”, pg. 69-73). Updegraff Press.
3. Peachey, Paul. (2001). Leaving and Clinging: the Human Significance of the Conjugal Union (ch. 1: “The Marital Bond as the Human Molecule”, pgs. 3-20). University Press of America.
4. Sexton, Thomas, Weeks, Gerald, and Robbins, Michael. (2003). Handbook of Family Therapy (pg. 6). Routledge.
5. Goldenberg, Herbert and Goldenberg, Irene. (2007). Family Therapy: An Overview (entropy, pgs. 91, 98, 298). Brooks Cole.
● Winiarski, Leon. (1898). “Theory of Property and Family: Essay on Social Mechanics” (Italian → English) ("La Teoria Della Proprieta E Della Famiclia: Saggi Sulla Meccanica Sociale Pura") (pgs. 572-594) In: Rivista Italiana di Sociologia (Italian Journal of Sociology), Volume 3. Fratelli Bocca, 1899.
● Zuk, Gerald H. and Boszormenyi-Nagy, Ivan. (1967). Family Therapy and Disturbed Families (Entropy and Family Therapy: Speculations on Psychic Energy, Thermodynamics, and Family Interpsychic Communication, pgs. 85-92). Science and Behavior Books.
● Family – Wikipedia.