The Atom's physical appearance is on his appearance in the Arrowverse TV shows Arrow and its spin-off DC's Legends of Tomorrow.
Scientist Ray Palmer used the radiation of a dwarf star to shrink matter, but the shrunk objects exploded shortly after, so he deemed it a failure. When some friends got trapped in a cave, Palmer used the radiation to shrink himself and save them. As he luckily didn't explode, Palmer became the Atom to use his new powers against crime.
In the CW series Arrow, Ray Palmer buys Queen Consolidated after Oliver loses his position as CEO.
He develops an exosuit and begins his activity as a vigilante as "The Atom". However, one day, trying out a new feature in his suit, Ray accidentally caused an explosion and manages to shrink himself in the process.
He is later taken by H.I.V.E. and is rescued by Team Arrow.
In the show spinoff DC's Legends of Tomorrow, Ray is asked to join a team led by Time Master Rip Hunter to defeat the immortal villain Vandal Savage.