Originally from Earth-9, Arthur Thompson was the only survivor of an American government program of research started in the 1960's, exposing test subjects to radiation. Thompson, who gained atomic powers. He entered the public eye in 1962, when he intervened in the Cuban Missile Crisis, saving untold millions of lives. Or that's what everyone believed.
He became famous as the world's first superhero, but he was never close to the public eye, because the same radiation than give him powers also mutated physically. Eventually he retiring from heroics and didn't involved in world troubles anymore, moving himself to the moon whit his family.
His son and grandson inherit his powers to keep his heroic legacy, with different results: his son was killed by the Fatal Five and when the third Atom stopped the FF, he discovered the greatest sin of the original Atom: he didn't helped to stop the '62 missile crisis, he started it.
Pressed by his grandchild to face the consequences of his actions, Thompson Sr. agreed to confess his responsibility to the world and declare in the USA Congress. But before he could continued his declaration he was killed by the Question, of the recently formed JLA, a team of American agents in charge of control the growing meta-human powers.
His death was brief. The Green Lantern bring him back to help the reunited Superman, Batman and Wonder Woman and stop the evil JLA. When they made it and take their enemies name as own, is most likely than Thompson returned to the land of the dead.
Because of the radiation exposure, Arthur Thompson can flight, have super force and can shoot concentrated radiation through his hands. He also can fight, control gravity, manipulate radiation, travel interstellar distances, among other atomic abilities. He passed these skills to his linage.