That’s why uranium is element number 92 on the periodic chart.
However, after two protons are ejected in an alpha particle, the nucleus then has only 90 protons, so the uranium has changed into element number 90, which is thorium.
The nucleus of thorium is still unstable, so it decays radioactively.
However, it decays by successively splitting and ejecting two electrons, known as beta particles, while gaining two extra protons, giving it a total of 92 again.
The picture is a little more complicated at the bottom end of this decay chain.
So the thorium has changed back into the element number 92, uranium. Whereas the original parent uranium atom had 146 neutrons and 92 protons in its nucleus (called uranium-238), this uranium atom has only 142 neutrons and 92 protons (called uranium-234).
The nucleus of this uranium-234 atom is still unstable, so it decays.
The main decay path is depicted with solid red arrows.
And the main eight alpha-decaying atoms are shown in red. As each of the alphaparticles is “fired” from the nucleus, it has a different energy.