Atom and Octavia first met in Earth Skills, when Bellamy asked Atom to watch over Octavia. At first, Atom viewed Octavia as a brat and took no interest in her at all. He later explains to her that Bellamy loves her very much and because of that she isn't a "basket case". He proceeds to say that he didn't have it worse than her, but since she has somebody (Bellamy) that would do anything for her he is jealous. Octavia later leaves camp and Atom follows her. They end up in part of the forest where beautiful "glowing" butterflies live. They end up passionately kissing each other. Later on, they are seen kissing again by the campfire. Bellamy sees them this time, though, and punishes Atom for disobeying him.
In Earth Kills, Atom blows off Octavia when she asks him to go see the butterflies with her again because he was punished by Bellamy when caught kissing her the previous night. Octavia later finds out Bellamy punished Atom and tells him that he can't keep everyone away from her, and that she's paying the price because Atom is ignoring her. Atom later gets caught in the acid fog and is suffering badly; his eyes are a pale, grey color and his skin is destroyed from where the fog touched it. Clarke mercy kills Atom once Bellamy can't bring himself to do it. Later that night, Octavia finds out about Atom's death and is deeply hurt by it.