1. NUCLEAR MASS AND ENERGY Physics 12

2. Clip of the day:  Minutephysics…on Einstein and uncertainty principle  http://www.youtube.com/watch?v=hSgIDgGpRpk http://www.youtube.com/watch?v=hSgIDgGpRpk  http://www.youtube.com/watch?v=7vc- Uvp3vwg&list=PL908547EAA7E4AE74 http://www.youtube.com/watch?v=7vc- Uvp3vwg&list=PL908547EAA7E4AE74

3. Protons, Neutrons and Electrons  The atom is composed of three subatomic particles:  A nucleon is a proton or neutron ParticleCharge (in C) SymbolMass (in kg) Electron-1.602x10 -19 e-e- 9.109 56x10 -31 Proton1.602x10 -19 p+p+ 1.672 614x10 -27 Neutron0n0n0 1.674 920x10 -27

4. Atomic Nucleus  Atom described using:  X – atomic symbol  A – atomic mass number (nucleon number)  Z – atomic number  Number of protons and electrons = Z  Number of neutrons = A - Z

5. Strong Nuclear Force  The electrostatic forces inside a nucleus would rip it apart if there was not another force  The strong force’s main job is to hold together the subatomic particles of the nucleus  By the end of the 1930’s physicists had determined that nucleons attract each other  This is the strongest force in the known universe which works at very close range

6. Stability and the Nucleus:  Although the Strong Nuclear Force is strong enough to hold a small nucleus together, as the size of the nucleus becomes larger, the electrostatic forces (repulsion between protons) begin to become more important  As a result, if we consider various nuclei based on their Atomic Number and Neutron Number we get the following result:

7. Nuclides and Isotopes  Nuclides are different combinations of nucleons  Isotopes occur when an element (specific Atomic Number) has different numbers of neutrons (different Atomic Mass Numbers)  For example, there are three common isotopes of hydrogen:

8. Nuclear Binding Energy  The energy to separate all the nucleons in a nucleus is called the binding energy  Comparison:  It takes 13.6eV to separate an electron from a hydrogen atom  However, it takes more than 20MeV to separate a neutron from a helium-4 atom

9. Fusion and Fission  Fission is the splitting of an atom into two or more smaller ones  Fusion is the fusing of two or more smaller atoms into a larger one.

10. Mass Defect  Imagine if we were able to apply the 20MeV required to separate a neutron from helium-4, what would happen to it?  Back to Einstein’s Special Theory of Relativity and the fact that mass and energy are equivalent……  The mass of helium-4 (2p, 2n) is smaller than that of helium-3 (2p, 1n) and a neutron separately  The energy that was added to remove the neutron was converted into mass!  Mass defect = the difference between the mass of a nuclide and the sum of the masses of its constituents

11. Atomic Mass Unit (u)  When dealing with nucleons, it is often more useful to deal with mass in Atomic Mass Units (u) instead of kilograms as the masses are very small ParticleMass (in kg) Mass (in u) Electron9.109 56x10 -31 0.000 549 Proton1.672 614x10 -27 1.007 276 Neutron1.674 920x10 -27 1.008 665

12. Example #1:  Determine the binding energy in electron volts and joules for an iron-56 nucleus given that the nuclear mass is 55.9206u

13.  Then divide by the number of nucleons to get binding energy per nucleon: = 7.88x10 -11 J x 1eV = 4.924x10 8 eV 1.6x10 -19 J Convert u to kg: 0.5285u x 1.6605 x10 -27 kg = 8.77 x 10 -11 kg

14. Try it  Page 904  1-3  Page 905  3, 4, 6, 8