Transmutation (elemental) – The ability to turn an object from one element into another element. A person with the power of elemental transmutation could turn a bullet into hydrogen but could not turn a bullet into flowers unlike a person that can manipulate matter at the molecular level.
Cosmic King (DC) is a supervillain that has this power (DC Who’s Who V1 #5).
Doctor Alchemy (DC) is a supervillain that has this power (DC Who’s Who V1 #6).
Element Lad (DC) is a superhero with this power.
Mr. Element (DC) is a supervillain that has this power (DC Who’s Who V1 #15.
During the Silver Age, DC often had their heroes turned into elements (Detective Comics V1 #302, Blackhawk V1 #182-1, Detective Comics #294).
Genii (Marvel) is one of the Young Gods and transmutation is one of his powers in Spectacular Spider-Man Annual V1 #8.
Shimmer (DC) is a super villain with this power in DC Who’s Who V1 #20.