# A2 Physics, Radioactivity and Nuclear Energy, 9-10

### 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

$$V=\frac{Q}{4\pi\epsilon_{0}d}\nonumber \\$$

Hence,

$$E_{p}=\frac{Q}{4\pi\epsilon_{0}d}q\nonumber \\$$

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

$$d=\frac{Qq}{4\pi\epsilon_{0}E_{k}}\nonumber \\$$
• 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; $$6.02\times 10^{23}$$.