Z:#Neutrons Ratio

Is there a fixed ratio that can be used to calculate the number of neutrons in the nucleus of the most stable isotope of an element by knowing the atomic number?


Side question: if Z is the symbol for atomic number, are there symbols for the number of neutrons and/or atomic mass?




The symbols $Z$, $N$, and $A$ are typically used to describe a nucleus:


$Z$ : atomic number (= number of protons)


$N$ : neutron number


$A$ : mass number ($A = Z + N$)


There is no fixed $Z:N$ ratio throughout the whole periodic table for stable isotopes, but a *belt of stability* in the [Chart of Nuclides](http://www.nndc.bnl.gov/chart/).


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