This Week Fission and fusion

What made our world Atomic and Hydrogen bombs

Big bangs in small packages Nuclear power plants

The solution to the energy problem? Enrichment

What does it mean and why is it necessary? Toxic Disposal Radiation in everyday life

Limit your exposure (same for the stock market) Fusion: The Holy Grail

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Fission and fusionIf we make an atom from it’s constituents energy is released. This is called the binding energy.Iron is the most tightly bound so if we can make elements up to Iron by the fusion of light elements energy is released.Similarly if we can break a very heavy atom into lighter elements we also have energy release.

E = mc2

Mass is another form of energy a nucleus is lighter than the sum of the constituent particle masses

Fusion processes are how the sun produces energy and fission processes are how nuclear reactors produce energy

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Chain reactionIf a U235 atom is broken into two lighter elements energy is released.This is accomplished using a neutron. The process produces 3 neutrons which can then cause 3 more atoms to undergo fission.Without any control this chain reaction produces an enormous amount of energy in a fraction of a second. It is an atomic bomb.This process was first observed by Enrico Fermi in a laboratory under the Chicago University football stadium

U235

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Manhattan project and the bomb

In the early forties all the top scientists were taken to Los Alamos to design the atomic bomb.In order for a bomb to work there had to be sufficient Uranium but it could not be in one piece because it fission would spontaneously occur. The main challenges were to obtain enough U235 and to design a system to bring the uranium together with a neutron source when the bomb was dropped.

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The hydrogen (fusion) bombFusion gives a much bigger energy release than fission, Hydrogen bombs are designed to use an atomic bomb to provide the temperature and pressure so that fusion takes place. There is a central core of light elements surrounded by plutonium or uranium.

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EnrichmentNaturally occurring uranium is nearly all U238

U235 is the fissionable material. Complicated processes have been developed to increase the fraction of U235, this is enrichment.For a reactor the final fraction of U235 is about 3.5%. For the original bomb several kilograms were required.In a reactor some of the U238 is turned into plutonium which is also a fissionable material that is used in atomic bombs.

Plutonium is produced and can be obtained from the spent fuel rods

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Nuclear reactorsIn order to produce energy we need to have controlled fission.Fuel rods are inserted into a moderator, which is used to slow the neutrons down and not let them escape.Control rods are used to absorb neutrons.The control rods can be moved in and out to set the level of energy production including shutting the reactor off completely. The energy is taken out with a circulating liquid and steam is generated to drive a turbine.

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Toxic disposalMost of the elements produced in the fission process are highly radioactive. All materials inside the containment vessel also become highly radioactive.Disposal of this toxic nuclear waste is a very big problem since it must not enter the environment. There is controversy over a proposed waste disposal site at Yucca mountain. In 1986 operator error at the Chernobyl nuclear plant caused an uncontrolled chain reaction. The resulting release of radiation had effects over almost all the northern hemisphere and thyroid cancer rates have increased 10 fold in the Ukraine

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Radiation in everyday lifeWe are exposed to radiation at low levels from a number of sourcesCosmic raysRadon and other natural radioactivityMedical imagingConsumer products fire alarmsRadiation is also used to treat cancerPET scansAll kinds of medical tracers e.g. radioactive iodineUsed in industry to control thickness of rolled steel and paperSterilization of food

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Fusion as an energy sourceEvery second the sun turns 600 million tons of hydrogen into 596 million tons of helium (with 4 million tons transformed into luminous energy via E=mc2).To emulate the sun we need extremely high temperatures and pressures to squeeze light elements together.Livermore National lab has tried to do this using high power laser beams to compress pellets of deuterium and tritium.There is a new international effort based in France called ITER to try and produce controlled fusion.

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Summary of Chapters 18 and 19n # electrons 1 (2L+1) x 2 = 22 (2L+1) x 2 = 83 (2L+1) x 2 = 18

The accelerating voltage is typically 30,000 volts and X rays are produced as the beam hits the screen

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The nucleus

Chain reaction E = mc2

DecayPeriodic table

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Questions Chapter 18

Q11 Assuming that cathode rays are a beam of charged particles, how could you demonstrate that these particles are negatively charged?

Q13 Would you expect X rays to be produced by a television picture tube?

By deflection in a magnetic field

Yes the accelerated electrons hitting the screen do produce X rays

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Q14 If the electron beam in a television tube is striking just one point on the screen at a time, how can we get a full picture?

Q20 What role did Rutherford’s scattering experiment play in our developing understanding of atomic structure?

Because the beam sweeps over the screen 30 times/second and we do not see this motion because of the speed and persistence of vision.

Rutherford discovered the nucleus made up of protons and neutrons surrounded at a large distance by electrons. The atom was not of uniform density like a marshmallow

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Q23 Does the spectrum of hydrogen consist of randomly spaced wavelengths or is there a pattern to the spacing?

There is unique pattern corresponding to the allowed energy levels. This allows the observation of hydrogen in deep space and the exansion of space by the shift to the red in the pattern

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Ch 18 E 6How many electrons are required to produce 1 microcoulomb of negative charge?

e = 1.6 x 10-19C1 microcoulomb = 10-6C = nen = 10-6/(1.6 x 10-19) = 6.25 x 1012 electrons

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Ch 18 E 8Use the Rydberg formula to find the wavelength of the line in the Balmer series of the hydrogen spectrum for m = 3.

R = 1.047 x 107 m-1

1/λ = R(1/n2 – 1/m2) , for Balmer series n = 21/λ = (1.097 x 107)(1/4 – 1/9) = 1523611 m-1

λ = 656.3 nm

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Ch 18 E 10Suppose a photon has wavelength λ = 520 nma) What is frequency of photon?b) What is photon’s energy in Joules?

a) c = fλ , f = c/λ = (3 x 108)/(520 x 10-19) = 5.769 x 1014Hz

b) E = hf = (6.626 x 10-34)(5.769 x 1014) = 3.823 x 10-19J

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Ch 18 E 12An electron in the hydrogen atom jumps from an orbit inwhich the energy is 1.89 eV higher than the energy ofthe final lower-energy orbit.a) What is the frequency of photon emitted in this

transition? (h = 4.14 x 10-15eVs)b) What is the wavelength of emitted photon?

b) c = λf , λ = c/f = (3 x 108)/(4.565 x 1014) = 6.571nm

a)

ΔE = hf , f = ΔE/h = 1.89/(4.14 x 10-15) = 4.565 x 1014 Hz

} ΔE = 1.89 eV

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Ch 18 CP 2E = 0 (n = ∞)

E4 = -0.85eV (n = 4)

E3 = -1.51eV (n = 3)

E2 = -3.4eV (n = 2)

E1 = -13.6eV (n = 1)

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Ch 18 CP 2 (cont)Looking at above Energy diagram for Hydrogen atom,(and remembering that the energies are all negative asa result of the negative potential energy for two chargesinteracting with opposite signs)a) Which transition in Balmer series (transition to n=2)

produces the largest wavelength photon?b) What is energy difference for the two energy levels in this

transition?c) What is frequency & wavelength of photon emitted in this

transition?d) Similarly find frequency & wavelength of photon with

longest wavelength in Lyman Series (transition to n=1).

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Ch 18 CP 2 (cont)

b) E3 = -1.51eV & E2 = -3.4eV

ΔE = -1.89eV (the electron loses this energy)

c) The electron loses 1.89eV. By energy conservation the photon gains 1.89eV.

E = hf , f = E/h = 1.89/(4.14 x 10-15) = 4.565 x 1014Hz

c = λf , λ = c/f = (3 x 108)/(4.565 x 1014) = 657.1 nm

a) 1/λ = R(1/n2 – 1/m2) , Balmer: n=2

1/λ = R(1/4 – 1/m2) , We want λ to be largest possible, that means 1/λis smallest possible. Compare m=3 and m=4.

m=3: 1/4 – 1/32 = 1/4 -1/9 = 5/36

m=4: 1/4 – 1/42 = 1/4 – 1/16 = 3/165/36 < 3/16}

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Ch 18 CP 2 (cont)

d) By the same argument conducted in part (a) we know we want the transition from m=2 to n=1.

For the electron ΔE = (-13.6eV) – (-3.4eV) = -10.2eV

The electron loses 10.2eV so photon gains 10.2eV

f = E/h = 10.2/(4.14 x 10-15) = 2.464 x 1015Hz

λ = c/f = (3 x 108)/(2.464 x 1015) = 121.8nm

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Ch 18 CP 4An electron (m = 9.1 x 10-31kg) moves with velocityv = 1500m/sa) What is the electron’s momentum?b) What is the electron’s deBroglie wavelength?c) How does this wavelength compare to visible light?

a) P = mv = (9.1 x 10-31)(1500) = 1.365 x 10-27 kg.m/s

b) λ = h/P = (6.626 x 10-34)/(1.365 x 10-27) = 485.4 nm

c) Blue light has λ = 440-490 nm. Electron has the same wavelength as blue light.

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Questions Chapter 19

Q1 In 1919, Rutherford bombarded a sample of nitrogen gas with a beam of alpha particles.A. In addition to alpha particles, what other particle emerged from the nitrogen gas in this experiment?B. What conclusion did Rutherford draw from this experiment? Explain.

Q3 Is it possible for two atoms of the same chemical element to have different masses?

Yes by adding or subtracting neutrons e.g. C12 and C14

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Q4 Is it possible for atoms of the same chemical element to have different chemical properties?

Q5 Which number, the mass number or the atomic number, determines the chemical properties of an element?

Q7 In a nuclear reaction, can the total mass of the products of the reaction be less than the total mass of the reactants?

No the chemical properties are determined by the electrons

The atomic number gives the number of electrons

Yes. E = mc2 and mass can be turned into energy

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Q13 In a time equal to two half-lives of a radioactive isotope, would you expect all of that isotope to have decayed?

Q15 Chemical reactions and nuclear reactions can both release energy. On the average, would you expect the energy released per unit of mass in a chemical reaction to be greater than, equal to, or less than what is released in nuclear reaction?

NO. In one half life ½ decay so in two half lives ¼ would be left

Chemical reactions involve changes in the electron energy levels and these are very low energy compared to nuclear reactions

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Q17 Suppose that you light a match to a mixture of oxygen and hydrogen, which then reacts explosively to form water. Is this a chemical reaction or a nuclear reaction?

Q18 The most common isotope of uranium is uranium-238. Is this the isotope that is most likely to undergo fission?

It is a chemical reaction in which two hydrogen atoms and one oxygen atom join.

NO U235

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Q19 What property of the fission reaction leads to the possibility of a chain reaction?

Q21 Do the control rods in a nuclear reactor absorb or emit neutrons?

The emittance of more than one neutron.

They absorb neutrons

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Q22 If you wanted to slow down the chain reaction in a nuclear reactor, would you remove or insert the control rods?

Q28 How does nuclear fusion differ from nuclear fission?

Insert

Fission is breaking a very heavy element into two lighter elementsFusion is joining two very light elements into on heavier elementBoth produce energy. Fusion produces the most and is cleaner but much more difficult

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Q29 Is nuclear fission the main process involved in the energy generated in the sun?

Q31 Which can produce larger yields of energy, a fission weapon or a fusion weapon?

No, the process is fusion.

A fusion weapon. The original bomb was a fission bomb. The hydrogen bomb uses a fission bomb to trigger a fusion bomb.

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Ch 19 E 294Pu239 is an isotope of Plutonium produced in nuclear reactors.a) How many protons in the nucleus of the isotope?b) How many neutrons in the nucleus of the isotope?

a) Atomic number = # of protons = 94

b) Mass number = # of protons + # of neutrons = 239# of neutrons = mass number - # of protons = 239-94 = 145

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Ch 19 E 6The fission fragment of iodine – 131 undergoes negative Beta Decay. Complete the reaction equation and identity the daughter nucleus.

53I131 → aXm + -1e° + oν--°unknown

Conserve mass number: 131 = m + 0 + 0, m=131

Conserve atomic number: 53 = 9 + -1 + 0, a = 54

54X131 = 54Xe131

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Ch 19 E 10

How many half lives must go by for radioactivity of a give isotope to drop to:a) one-sixteenth its original value?b) one-sixty fourth its original value?

a) In one half life, radioactivity drops by ½.1/16 = 1/2∙1/2∙1/2∙1/2 → four half lives

b) 1/64 = 1/2∙1/2∙1/2∙1/2∙1/2∙1/2 → six half lives

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Ch 19 E 12Suppose two deuterium nuclei (1H2 fuse in a reaction that emits a neutron. Complete the reaction equation and identify the resulting nucleus.

1H2 + 1H2 → aXm + on1

Conserve mass number 2 + 2 = m + 1, m = 3

Conserve atomic number 1 + 1 = a + 0, a = 2

2X3 = 2He3

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Ch 19 CP 4Nuclear power has been a source of controversy over the last few decades. Although the use of nuclear power has grown during this time, we still get more than half our power by burning fossil fuels. Discuss the different environmental & economic impacts of these power sources:a) Burning fossil fuels produces CO2 as a natural byproduct, a gas that

contributes to the greenhouse effect and to global warming. Is this a problem with nuclear power?

b) What environmental problems are associated with nuclear power that are not present in the burning of fossil fuels?

c) What environmental problems are associated with burning fossil fuels that are not present with nuclear power?

d) If alternatives were not available, which power source would you choose to develop further?

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CH 19 CP 4 cont.

a) No. CO2 is not created in the nuclear reaction associated with the fission of Uranium.

b) Disposal of nuclear wastes. Reactor safety. Depleted uranium fuelrods must be buried as they are still radioactive. Complex processesassociated with nuclear reaction and reactor stability lend nuclearreactors to events like that at Chernobyl!

c) Availability of fossil fuels. Green house gasses and other pollutants.

d) Nuclear power.