We’ve noted that fewer than 10% of the known nuclei are stable (i.e., not radioactive). Because nuclei are characterized by two independent numbers, $N$ and $Z$, it is useful to show the known nuclei on a plot of neutron number $N$ versus proton number $Z$. FIGURE 42.4 shows such a plot. Stable nuclei are represented by blue diamonds and unstable, radioactive nuclei by red dots.

FIGURE 42.4 Stable and unstable nuclei shown on a plot of neutron number $N$ versus proton number $Z$.

We can make several observations from this graph:

• The stable nuclei cluster very close to the curve called the line of stability.

• There are no stable nuclei with $Z>82$ (lead).

• Unstable nuclei are in bands along both sides of the line of stability.

• The lightest elements, with $Z<16$, are stable when $N\approx Z$. The familiar elements ${}^4\text{He}\text{,}{}^{12}\text{C}$, and ${}^{16}\text{O}$ all have equal numbers of protons and neutrons.

• As $Z$ increases, the number of neutrons needed for stability grows increasingly larger than the number of protons. The $N\text{/}Z$ ratio is $\approx 1.2$ at $Z=40$ but has grown to $\approx 1.5$ at $Z=80$.