Isotopes, isobars, and isotones

Nuclei can be classified and compared along three different axes, depending on which of $Z$, $A$ or $N$ they share:

Isotopes are atoms of the same element (same atomic number $Z$) but with different mass numbers $A$ - that is, the same number of protons but a different number of neutrons. For example, hydrogen has three isotopes: ${}^{1}_{1}H$ (ordinary hydrogen, no neutrons), ${}^{2}_{1}H$ (deuterium, 1 neutron), and ${}^{3}_{1}H$ (tritium, 2 neutrons) - all three have exactly one proton. Similarly, carbon has four main naturally occurring isotopes, ${}^{11}C$, ${}^{12}C$, ${}^{13}C$ and ${}^{14}C$, present in wildly different natural abundances: about 98.9% of natural carbon is $^{12}C$, about 1.1% is $^{13}C$, and only about $0.0001\%$ is the radioactive isotope $^{14}C$ (the other, $^{11}C$, does not occur naturally and can only be produced artificially in the laboratory or by cosmic rays). Because chemical behaviour is governed entirely by the electrons - which in turn is governed by $Z$, not $A$ - all isotopes of an element share the same chemical properties and sit in the same single position in the periodic table. …