The Atom
There have been four characters who have shared the Atom codename. The original Golden Age Atom, Al Pratt, was created by Ben Flinton and Bill O'Connor and first appeared in All-American Publications' All-American Comics #19 (Oct. 1940). The second Atom was the Silver Age Atom, Ray Palmer, who first appeared in 1961. The third Atom, Adam Cray, was a minor character present in Suicide Squad stories. The fourth Atom, Ryan Choi, debuted in a new Atom series in August 2006.
The Atom has been the star of multiple solo series, and three of the four have appeared as members of various superhero teams, such as the Justice Society of America, the Justice League, and the Suicide Squad.
The Atom introduced during the Silver Age of comic books in Showcase #34 (1961) is physicist and university professor Ray Palmer (named for real-life science fiction writer Raymond A. Palmer, who was himself quite short). Using a mass of white dwarf star matter, he fashioned a lens which allowed him to shrink down to subatomic size. Originally, his size and molecular density abilities derived from the white dwarf star material of his costume, controlled by mechanisms in his belt, and later by controls in the palms of his gloves. Much later, he gained the innate equivalent powers within his own body. After the events of Identity Crisis, Ray shrunk himself to microscopic size and disappeared. Finding him became a major theme of the Countdown year long series and crossover event.