Nuclear Stability You should be aware that: A nucleus can be naturally unstable Instability can be induced into a nucleus – for example if we bombard it with other particles As a result a nucleus can decay In doing so it might change from one type of an element to another – ISOTOPES The decay is spontaneous Why don’t all nuclei explode……… Strong Nuclear forces hold them together once they are close enough!

Proton number Z Neutron Number N N = Z Centre of distribution for stable nuclei decays by  - decay decays by  decay or  + decay

Less stable More stable More energy per nucleon Less energy per nucleon Fusion proton Neutron Binding energy Hydrogen nucleus Stable and Unstable Nuclei

Binding Energy is not the energy holding the atom together. It is the energy release which is brought about when nucleons combine. It is also the amount of energy we have to put in to split the atoms apart! Which nuclei will be more stable? The ones with less energy per nucleon – less energy = more stable They must have lost the most energy per nucleon; Therefore…. They must have the greatest binding energy. Which nuclei will be more stable? greatest binding energy Those which have the greatest binding energy. Since they do not stay as individual atoms but bind together!

z proton number 90 Binding energy per nucleon 1H1H 4 He 12 C 16 O 56 Fe Binds together very strongly 4 12  Nuclides will always try to become more stable.  This means reduce their overall energy.  In other words, increase their binding energy per nucleon.

z proton number 90 Binding energy per nucleon 1H1H 4 He 12 C 16 O 56 Fe Light nuclides can achieve this by Light nuclides can achieve this by:  undergoing fusion (“sticking together”) i.e. making heavier nuclei, such as in the Sun  reducing the energy per nucleon  increasing the binding energy per nucleon Heavy nuclei can achieve this by Heavy nuclei can achieve this by:  undergoing fission (breaking up) i.e. making lighter nuclei such as in a nuclear reactor  reducing the energy per nucleon  increasing the binding energy per nucleon This might explain why the majority of the Earth’s core is made up of Iron!!!

B E per nucleon Fission Fusion Z Light nuclides can achieve this by: moving up the curve. Heavy nuclei can achieve this by: moving back, yet up the curve.

TMFEOAT Because work has to be done in separating the particles, you will find that there is a mass difference between the mass of the atom and the mass of its separate constituents. e.g. The mass of a Ca- 40 is u 20 protons – u 20 neutrons – u 20 electrons – u The difference between the 2 values is u

If we now put our mass difference into TMFEOAT we get; E = mc 2 = u x (3 x 10 8)2 = 352 MeV = 8.8MeV per nucleon We are using the unified mass constant here as it is more usual when working with atoms.