Largest Z Number
Name: Hassan A.
Please, I would like to know The largest possible atomic number Z predicted by
theoretical physics. I would like to know why this value of Z, if known, represents the upper
atomic number ?
The largest stable Z is 92, Uranium. It is radioactive with a half life of several billion
years. Nuclei with higher Z (more protons) are unstable and generally decay quickly, though
neptunium has a half life of several million years.
The basic reason why higher Z nuclei are not stable is that protons are positively charged and
repel each other. The nuclear force holds them (and the neutrons) together, but the coulomb
force of repulsion finally becomes decisive.
Uranium contains more neutrons (143), which do not repel one another electrically, but nuclei
with more neutrons are also unstable due to quantum mechanical effects which are difficult to
explain. It is like adding electrons to atoms as Z increases; the first are in low energy
orbits, but as more are added (they cannot go into the same state due to the Pauli Exclusion
Principle) they are in higher energy states. In the nucleus, additional neutrons are put in
higher energy states until they have too high energy to be bound. If you try to add a 144th
neutron to uranium, it will not stick.
Best, Richard J. Plano Professor of Physics emeritus, Rutgers University
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Update: June 2012