In social physics, social atom is term used to describe the individual person as a human particle in a statistical assembly of persons or simply as a metaphorical single "atom".
The term social atom, referring to a person, was first used by English writer William Adams in his 1903 book Memoirs of a Social Atom.  In the opening sentence, Adams declares “I call myself a Social Atom—a small speck of the surface of society.”  In modern terms, according to the science of human chemistry, this sentence translates to the effect that a person is a Human Molecule—a small 26-element point molecule attached to the substrate surface of the earth in the system of society. 
Another to have conceived of an intricate type of "social atom theory", between the years 1910 and 1951, was Romanian-born American group psychotherapist Jacob Moreno. 
The term was used in the title of the 2007 book The Social Atom by American theoretical physicist Mark Buchanan who argues that we should "think of people as the 'atoms' or elementary building blocks of the social world, the social atoms," so that we might begin to see large-scale patterns emerge at the level of groups, patterns that have little to do with the character of the individual people themselves. 
● Human chemical
● Human molecule
1. Biography: W.E. Adams (1832-1906) - chartist, republican, supporter of women's suffrage, and editor of the Newcastle Weekly Chronicle.
2. Adams, William Edwin. (1903). Memoirs of a Social Atom, (pg. xiii). Hutchinson & Co.
3. Thims, Libb. (2008). The Human Molecule, (preview). Morrisville, NC: LuLu.
4. (a) Zeig, Jeffrey, K. (1987). The Evolution of Psychotherapy (Section: “Psychodrama, Role Theory, and the Concept of the Social Atom” by Zerka T. Moreno (wife of Jacob), pgs. 341-366). Psychological Press.
(b) Moreno, J. L. (1951). Sociometry, Experimental Method and the Science of Society: An Approach to a New Political Orientation. Beacon, NY: Beacon House.
(c) Moreno, J. L. (1953). Who Shall Survive? Foundations of Sociometry, Group Psychotherapy and Sociodrama (student ed.). Roanoke, VA: Royal.
(d) Remer, Rory. (2001). “Social Atom Theory revisited”, International Journal of Action Methods: Psychodrama, Skill Training, and Role Playing, June 22.
(e) Remer, Rory. (2006). “Chaos Theory Links to Morenean Theory: A Synergistic Relationship”, 30 pgs. (Section: Social Atom Theory) (PDF). Heldref Publications.
(f) Remer, Rory. (2001). “Social Atom Theory revisited”, International Journal of Action Methods: Psychodrama, Skill Training, and Role Playing, June 22.
5. Buchanan, Mark. (2007). The Social Atom - why the Rich get Richer, Cheaters get Caught, and Your Neighbor Usually Looks Like You, (pgs. x-xi, 13). New York: Bloomsbury.
● Our Lives as Atoms, a New York Times Blog, by Mark Buchanan.
● The Social Atom: Physics and Human Affairs, Mark Buchanan, Associate Editor Complexus.
● The Social Atom - Blog by Mark Buchanan.
● Video Talk (on the social atom), by Mark Buchanan at Microsoft, 1 hour 10 minutes.