Atom History Essay An Atom is a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The Atom was theorized by a man named Democritus of Abdera, Thrace in between c.a 460 BC - ca. 370 B.C. Atom is derived from the Greek word “atomos”, which means to be uncuttable. He had a theory that everything is composed of “atoms.