Nuclear Chemistry

Chapter 21

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Review Chapter 3

Z = Atomic Number Atomic Number is the number of _______ .

Mass Number Number of _______ + ________

Average Atomic mass Weighted average of mass numbers of isotopes What is an isotope?

Why are electrons not included in the mass number?

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Hydrogen Isotopes

Protium (99.985%) 1 proton, 0 neutons, 1 electron

Deuterium (0.015%) (Heavy Water) __ proton, __ neutron, __ electron

Tritium (Rare) (Radioactive) __ proton, __ neutron, __ electron

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Mass of an atom?

Mass of 1 atom is 2.657 x 10-23

So use another method Carbon 12 atom weighs 12 Atomic Mass Units Atomic Mass Unit (AMU)

Easy to find the mass of an atom: Find mass number or atomic mass + attach AMU as the

units Example: Oxygen = 16 amu OR 15.9994 amu

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First some vocab

Nucleons – particles in the nucleus

Nuclide – another name for an atom Identified by the number of protons + neutrons

Nuclear Reaction – reaction that affects the nucleus of an atom

Transmutation – change in proton number Change in the identity of a nucleus Oxygen-16 transmutates via alpha emission to Carbon-

12

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Mass Defect

When nucleons bind together into a nucleus, they LOSE mass

Mass Defect – (sum of the masses of the protons + neutrons + electrons) – (atomic mass) Proton mass = 1.007 276 amu Neutron mass = 1.008 665 amu Electron mass = 0.000 5486 amu

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Find Mass Defect

Helium-4 atom (p. 681)

Helium atom = 2 protons, 2 neutrons, 2 electrons 2 protons = 2(1.007 276 amu) = 2.014 552 2 neutrons = 2(1.008 665 amu) = 2.017 330 2 electrons = 2(0.000 5486 amu) = 0.001 097 TOTAL: 4.032 979 amu

Periodic Table: 4.002 602

Mass Defect = 4.032 979 amu – 4.002 602 amu MASS DEFECT = 3.0377 x 10-2 amu

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Nuclear Binding Energy (NBE)

Definition – The energy released when a nucleus is formed from its nucleons

Mass defect can be converted to NBE by Einstein’s famous equation: E = mc2

E = energy m = mass c = speed of light = 3.00 x 108 m/s

Now we will find nuclear binding energy in the previous problem.

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Finding Nuclear Binding Energy

Mass defect for Helium-4 = 3.0377 x 10-2 amu

Step 1: Convert units: amu kg Conversion Factor: 1 amu = 1.6605 x 10-27 kg Calculation: (3.0377 x 10-2 amu) (1.6605 x 10-27 kg/amu) Mass = 5.0441 x 10-29 kg

E = mc2 & c = 3.00 x 108 m/s

E = (5.0441 x 10-29 kg) (3.00 x 108 m/s)2

E = 4.54 x 10-12 kg * m2/s2

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Nuclear Binding Energy

NBE is also the energy that must be input to break apart the nucleus into its constituent nucleons

Since energy is released when a nucleus forms, which is more stable the nucleus or the separated nucleons?

Nucleus, since energy is inversely proportional to stability Lower energy = MORE stability

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Another Problem

Calculate the nuclear binding energy of a Sulfur-32 atom

Step 1: Calculate the mass defect 16 protons (16*1.007276) + 16 neutrons (16*1.008665) + 16

electrons (16*0.0005486)= 16.116 416 + 16.138 64 + 0.0087776= 32.263 833 6= 32.263 83 Sig Figs !!!

Mass Defect = 32.26383 – 32.065 = 0.1988336= 0.199 amu Sig Figs !!!

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Another Problem (Page 2)

Step 2: Calculate the NBE

Mass in amu = 0.1988336 0.1988336 amu * (1.6605 x 10-27 kg/amu) Mass in kg = 3.301 632 x 10-28 kg

E = mc2

E = (3.301 632 x 10-28 kg)(3.00 x 108 m/s)2

E = 2.971 468 x 10-11 kg * m2/s2 = 2.97 x 10-11 kg * m2/s2

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Half-Life

Half-life – time required for ½ of a radioactive material to decay

Each radioactive nuclide has its own ½ life

Longer ½ life = more stable nuclide

After 1 Half-Life = 50% remain 2 Half-Lives = 25% remain 3 Half-Lives = 12.5% remain

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Potassium-40

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Half-Life = Math Problems

Phosphorous-32 has a ½ life of 14.3 days. How many milligrams (mg) remain after 57.2 days, if the sample began with 4.0 mg?

57.2 / 14.3 = 4 Half-Lives

4 Half-Lives = (1/2)(1/2)(1/2)(1/2) of original amount remains

1/16 of the original amount remains

4.0 * (1/16) = 0.25 mg remains

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Half-Life Problems (Page 2)

Complete problems from Packet “Practice Problems” which is next to the decay series page.

Complete PRACTICE Problems on pp. 689 in textbook

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Pp. 693 Bottom

Alpha particles cannot go through paper

Beta particles can go through paper but not aluminum

Gamma particles can go through both, but not lead or concrete

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Nuclear Fission

Nuclear Fission – heavy nucleus splits into more-stable nuclei of intermediate mass Mass will be converted to energy, usually a lot of energy Chain reaction – material that begins a reaction is also

one of the products so it can begin another reaction Critical Mass – minimum amount of nuclide that is

required to sustain a chain reaction Nuclear Power Generators use controlled-fission chain

reaction to produce energy Also produces unwanted radioactive nuclides Makes fish (and humans) glow!!

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Nuclear Weapons

Fission weapons were actually used against Nagasaki and Hiroshima at the end of WW2

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Nuclear Fusion

Low mass nuclei combine to form a heavier, more stable nucleus

Immense energy production

Source of energy for the Sun and many stars

Thermonuclear or H-bombs Fusion of Deuterium + Tritium 100 times power of atomic

bombs

¼ mile diameter & 320 feet deep This blast contaminated more US residents than any other activity Yucca Flats, NV