John Dubrovny was an average man, who was an average achiever throughout his life. He settled down with his wife and went about his daily routine of working as a Molecular engineer.
However, John wasn't happy with his average life, or his colleagues' perceptions of him. When all his colleagues had left for the night, John continued to work on his own side project, resurrecting the long thought dead science of Alchemy.
The device he created, however, worked in a different way to his expectations. When he was caught in the devices ray, he was transformed into an amalgamation of all the elements he wanted to manipulate.
Dubrovny then found he could become one or any combination of the elements found in the periodic table. Adopting the name of known elements as his name, Mr 103 (There were only 103 elements known at this point)
his activities brought him to the attention of the Doom Patrol. Dubrovny gave the Doom Patrol a difficult fight until the Chief managed to find a way to neutralize his power and capture him. Mr. 104 died in Russia while on a mission with The Suicide Squad. He was blasted from behind by a Rocket Red.
Later on Dubrovny developed an immunity to the weapon which had neutralised his power, escaping and going into hiding to continue his research on his condition. The research was underwritten by Beast Boy's then legal guardian, Nicholas Galtry. Beast Boy and Doom Patrol once again managed to defeat Dubrovny. With him dissipating into thin air, presumed dead.
Floating around in the upper atmosphere, Dubrovny reformed himself after encountering a powerful electrical storm. He renamed himself Mr 104, as a new element had been added to his repertoire. Mr 104 sought out Cliff Steele and Beast Boy again to exact his revenge, but he was eventually defeated by Beast Boy's very basic knowledge of chemistry.
Dubrovny appeared in Metropolis trying to kill his former lackey who ratted him out to the police. When Superman shows up to stop him he transforms into Kryptonite. Power Girl appears and the two easily take him down.
Dubrovny served on one mission with The Suicide Squad. He along with Psi, The Weasel, and The Thinker died on their mission. Mr. 104 dissipated into the atmosphere. He was able to reform later on and faced Beast Boy and his fellow Titans.
Mr. 104 has total control over almost the entire range of the periodic table of elements. He is able to physically transform himself into any one of those elements, or a combination of several, to create various weapons and traps. He is able to remain in total control of his transmuted molecules.