Isotopes, isobars, isotones

Atoms with the same $Z$ but different $N$ are called isotopes (from the Greek isos topos, “same place” in the periodic table). Having the same number of electrons, they have almost identical chemical properties, but very different physical properties and nuclear stability: one can be perfectly stable and another strongly radioactive. The distinction is fundamental, because it is almost always a single isotope, and not the element in general, that is fissile, radioactive, or useful for dating.

Alongside isotopes, for completeness, we define isobars (same $A$, different $Z$) and isotones (same $N$, different $Z$). But it is the isotope family that is the protagonist of applied nuclear physics.

Example — Three elements and their isotopes

• Hydrogen: ${}^1$H (a single proton, $99{,}98\%$), ${}^2$H deuterium (1 p + 1 n, $0{,}02\%$), ${}^3$H tritium (radioactive, mean life $\sim 12$ years).
• Uranium: ${}^{238}$U ($99{,}3\%$, mean life $4{,}5$ billion years), ${}^{235}$U ($0{,}7\%$, the only fissile one).
• Carbon: ${}^{12}$C (stable, $98{,}9\%$), ${}^{13}$C (stable, $1{,}1\%$), ${}^{14}$C (radioactive, $\tau_{1/2} = 5730$ years: the basis of archaeological dating).

Note the lesson of uranium: the usable nuclear fuel is only $0{,}7\%$ of natural uranium. To fuel a reactor or build a weapon, the ore must be enriched, that is, the fraction of ${}^{235}$U relative to ${}^{238}$U must be increased — a technically arduous process precisely because the two isotopes, chemical twins, differ only in mass.

Collegamenti

Argomenti: Fisica nucleare Oggetti: Nucleo atomico

Esercizi collegati: Esercizio svolto — decadimento del radon in cantina · Dalla legge fenomenologica all’esponenziale · Consumo di idrogeno del Sole