nucleon: anything you find in the nucleus, includes protons and neutrons
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Nucleon: anything you find in the nucleus, includes protons and neutrons
Number of protons determines element
Symbol: p+
Positive charge: +eMass = mp = 1.6726e-27 kgNumber of protons in nucleus =
atomic number, symbol = Z
Symbol is n0
No chargeMass = mn = 1.6749e-27 kgNumber of neutrons in nucleus =
neutron number, symbol NTotal 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, 12.000000u
1 u = 1/12 the mass of carbon-12 1 u = 1.660539e-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 =
mc2
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.