The Atom is a DC Comics superhero character, best known for being "the tiny one in the Justice League" (because of his shrinking powers - he's usually 6 inches tall.) He has starred in his own series many times. (Should not be confused with Captain Atom. Or Atom Smasher. Or a plain atom.)
Actually, there have been three superheroes named "The Atom". The first was Al Pratt. He first appeared in All-American Comics #19 (October, 1940). Created by Ben Flinton and Bill O'Conner. Pratt is a short guy who gets a boost in confidence after being trained in boxing, so he becomes the masked hero, The Atom (because he's small but powerful, get it?) He had no actual superpowers, but was instead a Badass Normal. (It was later retconned that exposure to radiation gave him super strength.) His series continued appearing All-American Comics until its 72nd issue (April, 1946). He was a founding member of the Justice Society, appearing in most of its adventures between 1940 and to 1951. He got a second series of his own in the backpages of Flash Comics. Lasting from issues #80 to #104 (February, 1947-February, 1949). The character went into hiatus in 1953, but was revived in 1963 along with other Justice Society members. He became the father of the superhero Damage and the godfather of the superhero Nuklon.
The second Atom was introduced during the Silver Age of comics and had nothing in common with the first other than the name. He first appeared in Showcase #34 (September, 1961), created by Gardner Fox and Gil Kane. Ray Palmer was a physicist who found a meteor made of "white dwarf star matter" that had shrinking powers (white dwarfs are "compressed" stars and so smaller than normal) so he used it to create a costume that gave him the power to shrink (all the way to subatomic size!) and also change his weight. Despite these unlikely powers he became a successful hero and joined the Justice League. Palmer is also known for having one of the worst romantic lives of any superhero: Jean Loring, his long-time girlfriend (then wife) not only cheated on him, she later went insane and even later became a supervillain. At one point Palmer had a series of Low Fantasy adventures in the Amazon Jungle involving a tribe of really tiny humans (in the Sword of the Atom miniseries.)
The Palmer Atom got a regular series, named Atom in July, 1962. It lasted in its original format to issue #38 (August, 1968). He was then paired up with Hawkman, for the remaining issues of his series. Atom and Hawkman lasted from issue #39 to #45 (October, 1968-October, 1969). Then the series was cancelled. Palmer got a 4-issue mini-series called Sword of the Atom (September - December, 1983). Three Sword of the Atom special issues appeared between 1984 and 1988. Palmer then got a regular series, Power of the Atom. Which lasted 18 issues, from August, 1988 to November, 1989. After that the character mostly had regular appearances in team books and crossovers.
The third Atom was Ryan Choi, an Asian American scientist who inherited Palmer's costume after he disappeared following the events of Identity Crisis. He first appeared in the one-shot DCU: Brave New World (2006), created by Gail Simone and Grant Morrison. He starred in his own series for 25 issues (September, 2006-September, 2008). He was then unceremoniously killed off, to the chagrin of his fans. Choi's death sparked a major controversy, and in 2011, it was announced that it would be undone and that he would star as the Atom in the high profile Justice League relaunch written by Geoff Johns and drawn by Jim Lee. Palmer appears as a major supporting character in Frankenstein: Agent of S.H.A.D.E., without the Atom identity.
The Ryan Choi Atom is the one most often seen in the Batman: The Brave And The Bold show, though Palmer also appeared in one episode (that was in fact based on the Sword Of The Atom series). Ray Palmer was the one in Justice League Unlimited, and referenced in Justice League (regular).
For completeness, we should mention Adam Cray, a senator's son who temporarily used Ray Palmer's belt and Atom codename in the Suicide Squad series. He too was killed off.
- Affirmative Action Legacy: Choi.
- Badass Bookworm: Palmer and Choi.
- Dating Catwoman: Choi was in a relationship with Wonder Woman villainess Giganta.
- Clothes Make the Superman: Palmer and Choi
- Heart Is an Awesome Power: shrinking and changing your weight
- I Love Nuclear Power: Pratt
- Incredible Shrinking Man: No kidding.
- Legacy Character: Choi and Cray to Palmer
- Mouse World: Often invoked, but only really applied in Sword Of The Atom (where Palmer was stuck at six inches tall.)
- Square-Cube Law: completely ignored.
- Squashed Flat: The focus of a Silver Age story, Fate of The Flattened Out Atom.
- Tiny Guy, Huge Girl: Choi had a relationship with Giganta.
- Token Minority: Choi was seen as this at first by some. That they killed him off as soon as his series failed doesn't help.