Modelling the stable nucleus and radioactive decay

Modelling the nucleus with biscuits

The basis of the relationship between proton number and neutron number for stable isotopes on the various form of decay can be demonstrated by making piles of two different kinds of biscuit side by side using biscuits of similar but not identical thicknesses. Twix and club biscuits work for this model, the twix are neutrons and the club protons. Build the nucleus by making two piles side by side so taht they support each other. Make a table of twix number against club number for towers of similar heights that support each other. A nucleus with too many club biscuits (protons) can be made more stable by removing two clubs (protons) and two twix (Neutrons) - an α particle. A nucleus with too many twix (neutrons) can be made more stable if we can turn the twix into a club (proton) - we can swap a twix for a club and a tic-tac (electron or β particle).

Simple physics explanation

Both protons and neutrons occupy energy levels, a bit like the electrons around the atom. The protons tend to push each other apart due to them all having a positive charge so the steps between the energy levels for protons are larger than the steps between energy for neutrons.

• Large nuclei (atomic number more than 50) with an excess of protons decay by α as this reduces the energy of the nucleus and brings the proton and neutron energies closer to each other, the energy change being largely the kinetic energy of the α particle.
• Nuclei with too many neutrons decay by β dacay. A neutron can turn into proton and an electron, the extra energy is largely converted into the kinetic energy of the β particle (strictly speaking, but beyond the GCSE, there is also another small difficult to detect paricle called the neutrino emitted with the β particle).
• Small nuclei with and excess of protons decay by β+ or electron capture which turns a proton into a neutron whilst emitting a positron (positive electron, or capturing an electron from the atom - the opposite of β decay).