What is Nuclides, size, and density? — Physics

Definition 25.1 University Physics — Year 3 · Chapter 25 — Nuclear Physics

A nuclide $^{A}_{Z}\text{X}$ holds $Z$ protons and $N = A - Z$ neutrons, bound by the strong interaction: intense ( ${\sim}100\,\times$ Coulomb at contact), short-ranged ( ${\sim}1\,\mathrm{fm}$ , felt only by touching neighbours), and blind to charge (it grips protons and neutrons alike). Scattering experiments give every nucleus the same recipe: radius

R = r_0A^{1/3} , \qquad r_0 \approx 1.2\,\mathrm{fm} ,

so volume grows like $A$ and the density is a universal $\rho \approx 2.3 \times 10^{17}\,\mathrm{kg}/\mathrm{m}^{3}$ — a raindrop of it would weigh like a fleet of tankers, and Chapter 27 will meet whole stars at this density. Isotopes share $Z$ but differ in $N$ : same chemistry, different nuclear fates.

The valley of stability. Light nuclei balance their two Fermi ladders (N = Z); heavy ones dilute their Coulomb bill with extra neutrons. A nuclide off the valley floor -decays back toward it — and fission fragments, born far above the line, are furiously radioactive for exactly this reason. — The valley of stability. Light nuclei balance their two Fermi ladders ( $N = Z$ ); heavy ones dilute their Coulomb bill with extra neutrons. A nuclide off the valley floor $\beta$ -decays back toward it — and fission fragments, born far above the line, are furiously radioactive for exactly this reason.

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