Since he was first developed by Steve Ditko in the 1960s, there have been several incarnations of Captain Atom.
Silver Age Captain Atom (1960-2010)
AKA Earth-4 Captain Atom
AKA Quantum Superman
Secret Identity/Alter Ego:
Allen Adam/N. Christopher Adam
First Appeared: Space Adventures #33 (Charlton Comics)

Earth-4 Captain Atom
(Final Crisis: Superman Beyond #1)
Bronze Age Captain Atom (1966-1986)
AKA Earth-4 Captain Atom
AKA Quantum Superman
Secret Identity/Alter Ego: Allen Adam/N. Christopher Adam/Nathaniel Adam
First Appeared: Captain Atom (v 1) #84 (Charlton Comics)

Charlton Bullseye Captain Atom
(Charlton Bullseye #7)
Charlton Bullseye Captain Atom (1982)
Secret Identity/Alter Ego:
Colonel John Adam
First Appeared: Charlton Bullseye #7 (Charlton)
This version only appeared in this issue, written by Benjamin Smith and Dan Reed and drawn by Dan Reed. He had a slightly altered origin and his Silver Age costume as well as a new name and rank.
Modern Age Captain Atom (1987-2011)Secret Identity/Alter Ego: Nathaniel Adam/Cameron Scott
First Appeared: Captain Atom (v 2) #1 (DC Comics)
Earth-22 Captain Atom (1996-2008)
Secret Identity/Alter Ego: Nathaniel Adam
First Appeared: Kingdom Come #1 (DC Comics)
Although Captain Atom appeared in this costume in the 2005 WildStorm mini-series, it was still the Modern Age Captain Atom.

DC Animated Universe Captain Atom
(Justice League Unlimited #1)
DC Animated Universe Captain Atom (2004-2013)
AKA Earth-16 Captain Atom
Secret Identity/Alter Ego: Nathaniel Adams/Allen Adams/Cameron Scott
First Appeared: Superman and Batman Magazine #8 (DC Comics)
Monarch 2 (1995-2008)
Secret Identity/Alter Ego: Nathaniel Adam
First Appeared: Extreme Justice #6 (DC Comics)
Monarch first appeared in Armageddon 2001 #1, but he was Hank Hall (Hawk). The Nathaniel Adam Monarch first surfaced in Extreme Justice #6, though it was later revealed that this “Nathaniel Adam” was a copy created by Hank Hall as a side-effect of the process that trapped him in the time stream after his and Atom’s adventures in the past.
L.A.W.: Living Assault Weapons Captain Atom (1999-2000)
AKA Modern Age Captain Atom
Secret Identity/Alter Ego:
Nathaniel Adam
First Appeared: L.A.W.: Living Assault Weapons #1 (DC Comics)
This was basically just a costume change for the Modern Age Captain Atom. It lasted only for this mini-series.
The Kryptonite Man (2005)
AKA Modern Age Captain Atom
Secret Identity/Alter Ego: Nathaniel Adam
First Appeared: Superman/Batman #20 (DC Comics)

General Adam
(Frankenstein and the Creatures of the Unknown #1)
Flashpoint Captain Atom (2011)
AKA General Nathaniel Adam
Secret Identity/Alter Ego: NA
First Appeared: Frankenstein and the Creatures of the Unknown #1 (DC Comics)
New 52 Captain Atom (2011-2017)
Secret Identity/Alter Ego: Nathaniel Adam
First Appeared: DC Comics: The New 52 #1 (DC Comics)
The post-Rebirth Captain Atom was created when the New 52 Captain Atom “committed suicide.” See The Fall and Rise of Captain Atom #1.
Post-Rebirth Captain Atom (2017 – Current)
Secret Identity/Alter Ego: Nathaniel Adam, Colonel Cameron Scott
First Appeared: The Fall and Rise of Captain Atom #2 (DC Comics)

Post-Rebirth Captain Atom

Captain Adam (DC: The New Frontier #5)
Captain Adam (2004)
AKA Earth-21 Nathaniel Adam
Secret Identity/Alter Ego: NA
First Appeared: DC: The New Frontier #5 (DC Comics)
Captain Adym (2011-current)
Secret Identity/Alter Ego: Nathaniel Adym
First Appeared: Legion Lost #0 (DC Comics)
In The Fury of Firestorm: The Nuclear Man #15, part of The New 52 Captain Atom was hurled into the future where he became Captain Adym of the Science Police. It is unclear at this writing if Captain Adym has any of his predecessors’ powers.
DC Universe Online Captain Atom (2012)
Secret Identity: Nathaniel Adam
First Appeared: DC Universe Online: Legends #22
Although this version of Captain Atom seems identical to the Modern Age Cap, this series was not part of the regular DC continuity.

Injustice Captain Atom
Injustice Captain Atom (2013)
Secret Identity: Nathaniel Christopher Adam
First Appeared: Injustice: Gods Among Us #18
This Captain Atom’s secret identity was not established in the comics as “Nathaniel Christopher Adam,” but the Injustice: Gods Among Us wiki does use this name.
SEE ALSO
BREACH * DR. MANHATTAN * GOLDEN AGE CAPTAIN ATOM
Captain Atom and all related characters are ©DC Comics, Inc.
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