Like Suns, atoms have a maximum size.
In the nucleus, after Calcium, all other nuclei need more neutrons than protons to ‘space’ the protons apart and to share the charge over all the nucleons present, otherwise the proton charges make the nucleus unstable and it splits to a lower element that is more stable and some fermions are ejected to balance the numbers.
By the time we get to Uranium, there are 92 protons but also 142, 143 or 146 Neutrons (U-234, U-235, U-238). These are very stable isotopes with half lives in the region of 100s of thousands (234) or 100s of millions of years (235, 238). They are also the most abundant, with U-238 making up over 99.27% of all Uranium.
Other isotopes range from U-215 to U-242 - most have half lives of milliseconds or less, some have up to a few days and one or 2 are as stable as the first three mentioned. These are all trace/infinitesimal amounts though.
Note that the only change in an isotope is the number of neutrons… this is nature’s padding.
Pushing
the analogy further: get the padding just right and there is no
jiggling in the ‘packet’ and things hardly ever get damaged… if the
padding is skimped or is too tight, then the whole package is liable to
break when jiggled or the packet is knocked.
So there is quite a range, but the most stable range is just around 234–238. In this stability graph, you can see a block of black squares in a row for Uranium (92 protons) and this corresponds to stable (long half-life isotopes).
Now
when we get to Copernicium we can see, for Z = 112, there should be six
stable isotopes (in the white circle, the island of stability), but no atoms of this element and atomic mass
have yet been created. This leads us into the field of Nuclear isomers
and spin, gamma decay and more. It is a well studied field by
professionals.
Suffice it to say, there is a reason why nature does not create certain elements, and only ‘man’ has attempted to create synthetic elements in high energy conditions - there are many configurations that are inherently unstable in nuclei, and the higher you go, the less stable elements appear, and the harder it is to create them. They also exist one atom at a time as we create them and then they decay to stable elements VERY quickly.
So nobody Knows ™; making these synthetic elements is very hit and miss, and they won’t last very long anyway. It might be possible to make an element with 200 protons and say 300 neutrons but its half-life will be probably in the order of femtoseconds.
It is worth mentioning here the three main forces involved:
Electromagnetic interaction - protons repel each other due to this.
Strong force
- at close distances, the mix of Protons and Neutrons interact with the
Strong force which overcomes the Proton charge repulsion.
Weak force
- this is the force which causes Neutrons to decay, and as Z approaches
100, the number of Neutrons present reach a limit in the nucleus where
the Weak force’s effects on Neutrons overcome the Strong force in the
Nucleus, thus causing Neutrons to decay where you would expect ‘more’ to
be better. This is why all elements past Uranium are radioactive.
Note: In Neutron Stars, Gravity overwhelms the other forces due to its extreme value.