Unit 12- Nuclear Chemistry Nucleon Positron Radioactive Radioisotope Tracer Transmutation Alpha particle Artificial transmutation Beta particle Fission Fusion Gamma ray Half-life Natural transmutation

Nuclear Reactions Involve the nucleus of an atom nucleons- protons and neutrons The nucleus opens, and protons and neutrons are rearranged This releases a tremendous amount of energy that holds the nucleus together – called binding energy Transmutation- when an atomic nucleus changes into the nucleus of another element “Normal” Chemical Reactions involve electrons, not protons and neutrons Strong force- what holds the nucleus together acts on particles that are close together

Nuclear stability Ratio of neutrons to protons Belt is located above a 1:1 ratio of neutrons to protons but below a 2:1 ratio

Nuclei with atomic numbers above 83 are unstable Therefore they are radioactive Too many neutrons, high neutron:proton ratio What is a radioactive atom? An unstable nucleus that spontaneously decays to form a more stable product Called a radioisotope

When a nucleus decays it will emit radiation in these possible forms: Alpha particle- α Helium nucleus Beta particle-β Has a 1- charge Positron- has a 1+ charge Similar to beta particle but opposite charge Gamma rays-γ Almost all nuclear decay emits some; similar to X-rays but has greater E~ high energy photons Table O!!

Penetrating ability of radiation Gamma

Deflection of Particles Negative - - - - - - - - - - - - - - - - - - - - - - - -   Positive + + + + + + + + + + + + + + + + + +

Alpha Emission (decay) 226 4 222 Ra He + Rn 88 2 86 Alpha Particle Radium-226 Radon-222 __________________________________________ Mass Number (P+N) 226 = 4 + 222 = Atomic Number (protons only) 88 2 + 86 Nuclide= a particular type of nucleus, characterized by a specific atomic number and nucleon number Nucleons (protons and neutrons) are rearranged but conserved The Law of Conservation of Matter is obeyed Atomic # decreases by 2, atomic mass decreases by 4 Table N

Alpha Emission Example:

Beta Emission (decay) U e + Np 239 239 U e + Np 92 -1 93 Beta Particle Uranium-239 Neptunium-239 __________________________________________ Mass Number (P+N) 239 = + 239 = Atomic Number (protons only) 92 -1 + 93 Law of Conservation is followed Atomic number goes up by one Atomic mass remains unchanged

Beta Emission Example:

Positron Emission (decay) 207 207 Po e + Bi 84 +1 83 Positron Bismuth-207 Polonium-207 __________________________________________ Mass Number (P+N) 207 = + 207 = + Atomic Number (protons only) 84 +1 83 Law of Conservation is followed Atomic number goes down by one Atomic mass remains unchanged

Positron Emission Example: Same result when a nucleus captures its least energetic electron K-capture

Gamma Emission The nucleus has energy levels just like electrons, but they involve a lot more energy. When the nucleus becomes more stable, a gamma ray may be released. This is a photon of high-energy light, and has no mass or charge. The atomic mass and number do not change with gamma. Gamma may occur by itself, or in conjunction with any other decay type

Gamma Emission Example:

Rules for Writing Nuclear Equations The mass on each side of the equation must be equal The charges on each side of the equation must be equal General Format X X+ Y A Z a z A-a Z-z

Types of transmutations Natural- The decay methods already discussed Alpha, beta, positron, gamma Natural; due to unstable nuclei Has 1 reactant Artificial- (2 types) 1- collide charged particle with target nucleus Accelerate protons or alpha particles to overcome repulsive forces from nucleus 1- the particles have to have enough E to overcome the repulsive forces between positively charged particle and positively charged nucleus this E is supplied by accelerating particles by magnetic or electrostatic fields, these machines are called synchotrons Most of the elements from 93 on up (the “transuranium” elements) were created using particle accelerators.

2- collide neutron with target nucleus Neutrons are obtained as nuclear reaction by-products; they are captured by nucleus’s strong force Used to prepare radioisotopes from stable nuclei Has 2 reactants 2- the reactions that donate the neutrons are similar to the reactions that create electricity. Neutron has no charge so it isn’t repelled from the nucleus, it’s captured by the forces that hold the protons and neutrons in nucleus Cyclotron linear accelerator

Nuclear Fission A heavy nucleus is split into two smaller nuclei with release of energy Mass is converted to energy (E = mc2) 1 235 92 141 n  1 + U Kr + Ba + 3 n 92 36 56 Fission- 2 s’s so it splits 1 into 2 Fusion – 1 s makes 1 fuses

Because neutrons are reactants and products, a chain reaction occurs Fission Neutron causes a large nucleus to split into smaller ones. Used in nuclear reactors. 10n + 23592U  + 9136Kr + 3(10n) 14256Ba Because neutrons are reactants and products, a chain reaction occurs

Nuclear Fission

Nuclear Fusion Two lighter nuclei are fused to form a heavier nucleus 1 4  H He + 2 4 e + energy (E = mc2) 1 2 -1 2  4 2 H He + energy (E = mc2) 1 2 20% of our electricity for our country is nuclear Waste is stored in spent fuel pools advantage: products aren’t highly radioactive like fission The process that occurs in stars and thermonuclear weapons (hydrogen bombs)

Fusion Combining of light nuclei to form heavier ones. Lots of energy required to start it, but lots of energy released. In Sun: The equations on this slide happen on the sun are still unable to do on Earth due to the tremendous temperature and pressure needed tor the reaction to take place 4(11H)  2(0+1e) 42He +

Energy in Nuclear Reactions Nuclear reactions result in a loss of a small amount of mass and, therefore, release a large amount of energy. E = mc2 Over 1 billion times more energy is produced in a nuclear reaction than in any other type of reaction The matter that’s converted into the energy is called the MASS DEFECT

Radioactive decay substances decay at a constant rate, regardless of temperature, pressure, etc Random event that can’t be predicted Table N shows mode of decay for some radioactive elements More to Table N than this! 

Half-life The time it takes for half the atoms in a given sample of an element to decay Each radioisotope has its own half-life Shorter half-life = more unstable

Half-life equations Table T in reference tables n= t/T n = number of half-lives t= time elapsed T= half-life Fraction of remaining: (1/2)n or (1/2)t/T Original mass= final mass x 2n

Decay of 20. 0 mg of Oxygen-15. What remains after 3 half-lives Decay of 20.0 mg of Oxygen-15. What remains after 3 half-lives? After 5 half-lives? Has a half life of 122 seconds 20 mg 10 mg 5 mg 2.5 mg

Going Forwards in Time How many grams of a 10.0 gram sample of I-131 (half-life of 8 days) will remain in 24 days? #HL = t/T = 24/8 = 3 Cut 10.0g in half 3 times: 5.00, 2.50, 1.25g

Going Backwards in Time How many grams of a 10.0 gram sample of I-131 (half-life of 8 days) would there have been 24 days ago? #HL = t/T = 24/8 = 3 Double 10.0g 3 times: 20.0, 40.0, 80.0 g

Radioactive Dating A sample of an ancient scroll contains 50% of the original steady-state concentration of C-14. How old is the scroll? 50% = 1 HL 1 HL X 5730 y/HL = 5730y

Uses of Radioisotopes Radioactive dating Carbon-14 for dating organic remains Potassium-40 for dating rocks Uranium-238 for determining the age of the earth

Radiation therapy for cancer Destroy cancer cells (can damage healthy cells) Irradiation of food Cobalt-60 produces gamma rays- kill bacteria Tracers Carbon-14 for photosynthesis Iodine-131 for thyroid disorders Technetium-99 for diagnosis of brain tumors Carbon-13 for real-time brain imaging (PET Scan) Why do these uses require short half-lives? Short half-lives so they don’t stay in body long

Other Uses of Radioactive Isotopes C-14: Carbon Dating (Finding Age of Organic Material) U-238: Geological Dating (Finding Age of Inorganic Material) U-235: Nuclear Power Plant Fuel P-31: Tracer in Plant Fertilizer I-131: Detection and Treatment of Thyroid Conditions Tc-99: Cancer Detection and Location Co-60: Cancer Treatment and destruction of bacteria Cs-137: Destruction of Bacteria