system
Given the following post, respond as if you're commenting on Reddit:
human
What is the upper limit of atom size (atomic number), and why?
gpt
I can only answer the why in a hand-waving way. Atomic number is equal to the number of protons (in the nucleus).  Nuclei are build up from neutrons and protons which attract via the relatively short-range strong nuclear force. Note that neutrons essentially do not attract each other via the strong nuclear force (hence, you do not find di-neutrons). In addition, protons repel each other via the long-range electrostatic interaction.  When you have relatively few protons, you can get enough neutrons together so that the effect of the strong interaction overcomes the electrostatic repulsion.  As the nucleus gets bigger and bigger, if you add one more proton, it feels the repulsive effect of all the protons present and the attractive effect of only the neighbouring neutrons due to the short range of the strong nuclear force.  There comes a point where this short range attraction can not make up for the long range electrostatic repulsion.