Which reaction converts an atom of one element to an atom of another eleme

This reaction is called nuclear transmutation. Atoms of one element can be changed into atoms of another element by a process which takes place through nuclear reactions whereby an outside particle reacts with a nucleus, or through what is known as radioactive decay, a process where no outside particle is required.

Not all radioactive decay or nuclear reactions result in transmutation but all transmutation is caused by either nuclear reaction or radioactive decay. For example when Uranium - 235 undergoes beta decay, it forms thorium-231