The nuclear explosions that occurred in Hiroshima and Nagasaki were not the result of splitting just one atom, but rather the fission (splitting) of a large number of atoms in the nuclear material. The destructive power of a nuclear bomb comes from the release of an enormous amount of energy in a very short period.
In these atomic bombs, such as the ones dropped during World War II, the explosive energy is generated by a chain reaction of nuclear fission. The bomb contains a critical mass of fissile material, typically enriched uranium-235 or plutonium-239. When a neutron is absorbed by a fissile atom, it becomes highly unstable and splits into two smaller atoms, releasing a significant amount of energy and additional neutrons. These newly released neutrons can then go on to trigger the fission of other fissile atoms in a chain reaction.
The critical mass of fissile material required to sustain a chain reaction is necessary to achieve a rapid and self-sustaining release of energy. In the Hiroshima and Nagasaki bombs, the chain reaction was initiated by conventional explosives that compressed and brought together subcritical masses of fissile material to form a supercritical mass.
The release of energy from the fission of a single atom is relatively small. It is the cumulative effect of a vast number of atoms undergoing fission in a chain reaction that leads to the massive release of energy in a nuclear explosion. The explosion occurs within a fraction of a second, with an immense amount of energy released in the form of a rapidly expanding shockwave, intense heat, and radiation.
To summarize, a nuclear explosion like those in Hiroshima and Nagasaki involves the fission of thousands or even millions of atoms in the nuclear material, rather than just the splitting of a single atom. The chain reaction amplifies the release of energy, resulting in a devastating and destructive blast.