 Nucleon: anything you find in the nucleus, includes protons and neutrons

 Number of protons determines element  Symbol: p +  Positive charge: +e  Mass = m p = e-27 kg  Number of protons in nucleus = atomic number, symbol = Z

 Symbol is n 0  No charge  Mass = m n = e-27 kg  Number of neutrons in nucleus = neutron number, symbol N  Total number of nucleons (protons + neutrons) in a nucleus is call the atomic mass number, symbol A A = Z + N

 Atoms of the same element can have different number of neutrons in the nucleus (even though same number of protons), called ISOTOPES  Isotopes react almost identically when compared to each other, but in physics we’re concerned with different isotopes  Masses on periodic table are weighted averages based on natural abundances

 Ex, since most carbon is carbon-12, the number is pretty close to 12

 The number of neutrons strongly affects the stability of the nucleus  In unstable isotopes, the number of neutrons partly determines the rate at which the nucleus decays and releases radiation

 Masses of atoms are sometimes given in atomic mass units (amu), which has the unit “u”  Not an SI unit, but measuring small things in kg can seem silly, so it’s common  Based on neutral carbon-12 atom, u  1 u = 1/12 the mass of carbon-12  1 u = e-27 kg

 Since the strong nuclear force holds nucleons together, energy must be added to separate them… this is binding energy  Separated nucleons have more energy  Nucleons bound in nucleus haven’t had energy added yet, so they have more energy

 Since separated nucleons have more energy, they must have more mass (energy is directly related to mass)  Nucleons bound in the nucleus have less energy and therefore less mass

 Mass defect = difference between the mass of the nucleus and its individual nucleons  Directly related to the binding energy added to break apart the nucleus

 1896: leaves uranium in a drawer with a photographic plate and accidentally identifies another part of the electromagnetic spectrum

 Isolated two other radioactive elements: polonium and radium  Put them under different stresses, but the elements always emitted radiation, so concluded radioactivity comes from deep within the atom (i.e., the nucleus)

 Results from the decay of an unstable nucleus  Decay happens because it results in a more stable nucleus

 Ernest Rutherford found 3 distinct forms of radiation & divided based on ability to pass through material and deflection in magnetic field  Alpha (α): could barely pass through a single sheet of paper. Deflected as a positive particle in a magnetic field.  Beta (β): can pass through about 3mm of aluminum. Deflected as a negative particle in a magnetic field. *  Gamma (γ): can pass through several centimeters of LEAD! Not deflected in a magnetic field.

 Alpha radiation is a Helium atom, but we call it an alpha particle since it comes from radiation  With protons and neutrons leaving the nucleus it gets smaller, often more stable  Alpha particle: charge +2e, since no electrons  Use conservation of nucleons to write-out decay

 Total mass of the daughter nucleus plus the alpha particle is less than the mass of the original nucleus  Missing mass was turned into energy : E = mc 2  Works with our understanding of conservation of mass and energy being interchangeable  Energy found mostly in kinetic energy of alpha particle and daughter nucleus moving away from one another

 A neutron falls apart and becomes a proton and an electron  Leaving electron is the beta particle  That’s why a neutrons mass is a little bigger than a protons

 Particles emitted are opposite from beta negative decay  Positive positron, sometimes called an anti electron (antimatter version of an electron)  Same mass as an electron, but positive charge

 Emits a form of EMR, not a particle =>much harder to stop (it’s pretty high-up in frequency of the EM spectrum)  Happens most often after alpha and beta decay  Nucleus has been through a lot and needs to release excess energy  Since it’s a release of energy A and Z stay the same

 Half life of an element: the time it will take half of the parent atoms to transmutate into something else  Through alpha or beta decays, or another process  Total number of atoms stays constant  Based on statistics

 The half life of C-14 is 5730 years. Explain what you would expect to happen over a long period of time.

 Activity measures the number of nuclei that decay per second  Measured in Becquerels (Bq) = decays/second.  Geiger counter clicking in movies measures the activity of the sample.  As time passes, the number of nuclei available decrease and sample activity does too

 You have 75 g of lead-212. If it has a half life of 10.6 hours, determine how long it will take until only 9.3 g remains.

 What do you think of when I say nuclear energy?

 There are 2 types of nuclear reactions that release energy  Fission  Fusion

 The process of causing a large nucleus (A > 120) to split into multiple smaller nuclei, releasing energy in the process.  Can start when large nuclei absorbs a neutron, causing it to become unstable to the point that it falls apart  Reaction that we use in nuclear power plants and early nuclear weapon  Pretty easy and cheap energy  Lots of nuclear waste stored for a long time

 The process of causing small nuclei to stick together into a larger nucleus, in the process releasing energy.  Process that drives our sun and all other suns  We can duplicate in a lab, but use more energy than we get out  Left over products are safe, so lots of research goes into trying to develop fusion reactors

 The most typical fuel used in a fission reactor is uranium-235.  1939: 4 German scientists discovered that uranium- 235 would become very unstable if it gained an extra neutron, forming uranium-236.  Uranium-236 is so unstable that a fraction of a second later it will split to form two smaller atoms, and in the process release energy.

 If one neutron gives rise to another reaction, the self sustaining reaction that results is called critical.  Each reaction leads to one reaction afterwards.  This is a “chain reaction”.  If 2+ neutrons give rise to more reactions, the increasing rate of reactions is called supercritical.  Each reaction leads to multiple reactions afterwards.  Generations of reactions increase exponentially

 There are a few situations when we want this to happen...  Nuclear bomb, since we want one reaction we kick off to result in a cascade of exponentially more and more reactions within a split second  When a nuclear power plant is first being started up ▪ Then stepped down to a critical reaction. ▪ If the nuclear reactor is melting down then supercritical reactions are BAD

 You need subcritical reactions  Less than a neutron gives rise to other reactions

 Reactors use control rods to control the rate of the reaction.  Made from elements such as boron and cadmium, control rods are very good at absorbing neutrons.  If a reaction is going supercritical, drop the control rods further into the core to absorb extra neutrons and the reaction slows.  If the reaction is going subcritical, pull the control rods out further, which lets more neutrons react and get more reactions going again.