Question

Which is more radioactive ${U_{235}}$ or ${U_{238}}$ ?

Answer 1

The given elements are isotopes of $Uranium$ . Isotopes have the same number of protons but different neutrons. ${U_{235}}$ can sustain a fission chain reaction. If the given isotopes can be transformed then they are unstable.

Complete answer:
$Uranium$ is a naturally occurring radioactive element. Its nucleus is unstable, so the element is in a constant state of decay, seeking a more stable arrangement. It was due to the element $Uranium$ it made possible the discovery of radioactivity. In $1897$ , when physicist Henri Becquerel left some $Uranium$ salts on a photographic plate as part of some research , the plate fogged up, indicating some sort of emission from $Uranium$ salts.
Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. When an element’s nucleus is unstable, it is considered as radioactive. There are three types of decay, namely alpha-decay, beta-decay and gamma-decay, depending upon the number of emitting particles or photons.
The nucleus of ${U_{235}}$ atom contains $92$ protons and $143$ neutrons, which gives it the atomic mass of $235$ . The ${U_{238}}$ nucleus also has $92$ protons but its neutrons are $146$ in numbers. Thus ${U_{238}}$ has three more neutrons than the ${U_{235}}$ and has a mass of $238$ . As we have learned about radioactivity, an unstable nucleus is constantly decaying to seek stable arrangement, so the element having less number of neutrons will be stable or more radioactive, which in this case is ${U_{235}}$ .

Note:
The $Uranium$ atom is the heaviest atom present in the natural environment. Its radioactivity is very low. Its very long life of several billion years has allowed $Uranium$ to be still present. $Uranium$ is used in power reactor operation and was used in the atomic bomb warfare of Hiroshima.