That name is also already taken. An atom bomb usually refers to the first generation of nuclear bombs that use only fission, e.g., those used in WWII. The hydrogen bomb refers to the second generation of nuclear bombs that use a chain reaction of nuclear fission and fusion to create bombs that are orders of magnitude more powerful.

For comparison, Fat Man was ~20 kilotons and the largest ever bomb (Tsar Bomba) was ~50,000 kilotons.