NUCLEAR MASS AND ENERGY Physics 12

Clip of the day:  Minutephysics…on Einstein and uncertainty principle   Uvp3vwg&list=PL908547EAA7E4AE74 Uvp3vwg&list=PL908547EAA7E4AE74

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-1.602x e-e x Proton1.602x p+p x Neutron0n0n x10 -27

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

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

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:

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:

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

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.

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

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) Electron x Proton x Neutron x

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

 Then divide by the number of nucleons to get binding energy per nucleon: = 7.88x J x 1eV = 4.924x10 8 eV 1.6x J Convert u to kg: u x x kg = 8.77 x kg

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