A2 Physics, Radioactivity and Nuclear Energy, 9-10

Radioactivity

Structure of the Nucleus

Rutherford Scattering

  • Alpha particles fired at gold foil

    • Foil thin so alpha particle only scattered once

    • Evacuated tube used

    • Zinc Sulfide screen showed scintillations

    • Alpha source produced a narrow beam to allow precise measurements

      • *

        Also monoenergetic

    • Most passed through

    • Some were scattered

    • Very few were reflected

    • Showed that the alpha particle was colliding with a dense, positively charged object, much more massive than \({}^{4}_{2}He\)

    • Allowed theoretical upper limit on size of gold nuclei to be calculated

The maximum radius of the gold nuclei is \(d\)

At a distance \(d\) from a charge \(Q\), the potential will be given by

\begin{equation} V=\frac{Q}{4\pi\epsilon_{0}d}\nonumber \\ \end{equation}

Hence,

\begin{equation} E_{p}=\frac{Q}{4\pi\epsilon_{0}d}q\nonumber \\ \end{equation}

Since this can be equated to \(E_{k}\) when the particle is stationary, \(d\) can be found:

\begin{equation} d=\frac{Qq}{4\pi\epsilon_{0}E_{k}}\nonumber \\ \end{equation}
  • Assumes head on collision, which was impossible to detect

  • Neglects the strong force

  • Neglects any recoil

However, the result did confirm that the atom is mostly empty space.

Model of the Atom

  • Electrons are quantised, and can only exist in given energy levels

  • Their position is best given by an electron cloud model

  • Nucleus contains protons and neutrons

  • \({}^{A}_{Z}X\), where \(A=\) the nucleon number, and \(Z=\) the proton number

  • Isotopes will have a different number of neutrons

The nucleon expressed as a mass in grams approximates to the mass of one mole of a given isotope.
One mole of a substance contains the same number of entities as there are atoms in 12g of carbon-12; \(6.02\times 10^{23}\).