Nuclear Chemistry Chapter 23

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

Nuclear Reactions vs. Normal Chemical Changes Nuclear reactions involve the nucleus The nucleus opens, and protons and neutrons are rearranged The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy “Normal” Chemical Reactions involve electrons, not protons and neutrons

23.1

Types of Radiation Beta (β) – an electron Alpha (ά) – a positively charged (+2) helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons Beta (β) – an electron Gamma (γ) – pure energy; called a ray rather than a particle

Other Nuclear Particles Neutron Positron – a positive electron Proton – usually referred to as hydrogen-1 Any other elemental isotope

Penetrating Ability

Geiger-Müller Counter 23.7

Geiger Counter Used to detect radioactive substances

X Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Mass Number X A Z Element Symbol Atomic Number 1p 1 1H or proton 1n neutron 0e -1 0b or electron 0e +1 0b or positron 4He 2 4a or a particle A 1 1 4 Z 1 -1 +1 2 23.1

Balancing Nuclear Equations Conserve mass number (A). The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants. 1n U 235 92 + Cs 138 55 Rb 96 37 + 2 235 + 1 = 138 + 96 + 2x1 Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants. 1n U 235 92 + Cs 138 55 Rb 96 37 + 2 92 + 0 = 55 + 37 + 2x0 23.1

212Po decays by alpha emission 212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po. 4He 2 4a or alpha particle - 212Po 4He + AX 84 2 Z 212 = 4 + A A = 208 84 = 2 + Z Z = 82 212Po 4He + 208Pb 84 2 82 23.1

Nuclear Stability and Radioactive Decay Beta decay 14C 14N + 0b + n 6 7 -1 Decrease # of neutrons by 1 40K 40Ca + 0b + n 19 20 -1 Increase # of protons by 1 1n 1p + 0b + n 1 -1 Positron decay 11C 11B + 0b + n 6 5 +1 Increase # of neutrons by 1 38K 38Ar + 0b + n 19 18 +1 Decrease # of protons by 1 1p 1n + 0b + n 1 +1 n and n have A = 0 and Z = 0 23.2

Nuclear Stability and Radioactive Decay Electron capture decay 37Ar + 0e 37Cl + n 18 17 -1 Increase # of neutrons by 1 55Fe + 0e 55Mn + n 26 25 -1 Decrease # of protons by 1 1p + 0e 1n + n 1 -1 Alpha decay Decrease # of neutrons by 2 212Po 4He + 208Pb 84 2 82 Decrease # of protons by 2 Spontaneous fission 252Cf 2125In + 21n 98 49 23.2

Learning Check What radioactive isotope is produced in the following bombardment of boron? 10B + 4He ? + 1n 5 2 0

Learning Check What radioactive isotope is produced in the following bombardment of boron? 10B + 4He 13N + 1n 5 2 7 0

Write Nuclear Equations! Write the nuclear equation for the beta emitter Co-60. 60Co 0e + 60Ni 27 -1 28

Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B. Reactions using neutrons are called g reactions because a g ray is usually emitted. Radioisotopes used in medicine are often made by g reactions.

Artificial Nuclear Reactions Example of a g reaction is production of radioactive 31P for use in studies of P uptake in the body. 3115P + 10n ---> 3215P + g

Transuranium Elements Elements beyond 92 (transuranium) made starting with an g reaction 23892U + 10n ---> 23992U + g 23992U ---> 23993Np + 0-1b 23993Np ---> 23994Pu + 0-1b

Nuclear Stability Certain numbers of neutrons and protons are extra stable n or p = 2, 8, 20, 50, 82 and 126 Like extra stable numbers of electrons in noble gases (e- = 2, 10, 18, 36, 54 and 86) Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons All isotopes of the elements with atomic numbers higher than 83 are radioactive All isotopes of Tc and Pm are radioactive 23.2

Band of Stability and Radioactive Decay

Stability of Nuclei Out of > 300 stable isotopes: N Even Odd Z 157 52 3115P Even Odd 50 5 21H, 63Li, 105B, 147N, 18073Ta 199F

Half-Life HALF-LIFE is the time that it takes for 1/2 a sample to decompose. The rate of a nuclear transformation depends only on the “reactant” concentration.

Half-Life Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234 After 3.6 days > 25 grams After 7.2 days > 12.5 grams After 10.8 days > 6.25 grams

Kinetics of Radioactive Decay N daughter rate = - DN Dt rate = lN DN Dt = lN - N = N0e(-lt) lnN = lnN0 - lt N = the number of atoms at time t N0 = the number of atoms at time t = 0 l is the decay constant (sometimes called k) Ln 2 = t½ l k = 23.3

Kinetics of Radioactive Decay [N] = [N]0exp(-lt) ln[N] = ln[N]0 - lt [N] ln [N] 23.3

Radiocarbon Dating 14N + 1n 14C + 1H 14C 14N + 0b + n t½ = 5730 years 6 14C 14N + 0b + n 6 7 -1 t½ = 5730 years Uranium-238 Dating 238U 206Pb + 8 4a + 6 0b 92 -1 82 2 t½ = 4.51 x 109 years 23.3

Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 31 hours?

Biological Effects of Radiation Radiation absorbed dose (rad) 1 rad = 1 x 10-5 J/g of material Roentgen equivalent for man (rem) 1 rem = 1 rad x Q Quality Factor g-ray = 1 b = 1 a = 20 23.8

Effects of Radiation

Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron) 2. Propagation. 236U fission releases neutrons that initiate other fissions 3. Termination.

Nuclear Fission

Nuclear Fission 235U + 1n 90Sr + 143Xe + 31n + Energy 92 54 38 Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2 Energy = 3.3 x 10-11J per 235U = 2.0 x 1013 J per mole 235U Combustion of 1 ton of coal = 5 x 107 J 23.5

Representation of a fission process.

Mass Defect Some of the mass can be converted into energy Shown by a very famous equation! E=mc2 Energy Mass Speed of light

BE = 9 x (p mass) + 10 x (n mass) – 19F mass Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. BE + 19F 91p + 101n 9 1 E = mc2 BE = 9 x (p mass) + 10 x (n mass) – 19F mass BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984 BE = 0.1587 amu 1 amu = 1.49 x 10-10 J BE = 2.37 x 10-11J binding energy per nucleon = binding energy number of nucleons = 2.37 x 10-11 J 19 nucleons = 1.25 x 10-12 J 23.2

Nuclear binding energy per nucleon vs Mass number nuclear stability 23.2

Nuclear Fission Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass. Non-critical Critical 23.5

Nuclear Fission & POWER Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear.

Diagram of a nuclear power plant

Annual Waste Production Nuclear Fission 35,000 tons SO2 4.5 x 106 tons CO2 1,000 MW coal-fired power plant 3.5 x 106 ft3 ash Annual Waste Production 1,000 MW nuclear power plant 70 ft3 vitrified waste 23.5

Hazards of the radioactivities in spent fuel compared to uranium ore Nuclear Fission Hazards of the radioactivities in spent fuel compared to uranium ore 23.5 From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG

Chemistry In Action: Nature’s Own Fission Reactor Natural Uranium 0.7202 % U-235 99.2798% U-238 Measured at Oklo 0.7171 % U-235

Fusion Nuclear Fusion small nuclei combine 2H + 3H 4He + 1n + 1 1 2 0 1 1 2 0 Occurs in the sun and other stars Energy

Tokamak magnetic plasma confinement Nuclear Fusion Fusion Reaction Energy Released 2H + 2H 3H + 1H 1 6.3 x 10-13 J 2H + 3H 4He + 1n 1 2 2.8 x 10-12 J 3.6 x 10-12 J 6Li + 2H 2 4He 3 1 2 Tokamak magnetic plasma confinement 23.6

Nuclear Fusion Fusion Excessive heat can not be contained Attempts at “cold” fusion have FAILED. “Hot” fusion is difficult to contain

Radioisotopes in Medicine 1 out of every 3 hospital patients will undergo a nuclear medicine procedure 24Na, t½ = 14.8 hr, b emitter, blood-flow tracer 131I, t½ = 14.8 hr, b emitter, thyroid gland activity 123I, t½ = 13.3 hr, g-ray emitter, brain imaging 18F, t½ = 1.8 hr, b+ emitter, positron emission tomography 99mTc, t½ = 6 hr, g-ray emitter, imaging agent Brain images with 123I-labeled compound 23.7

Chemistry In Action: Food Irradiation Dosage Effect Up to 100 kilorad Inhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables. 100 – 1000 kilorads Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit. 1000 to 10,000 kilorads Sterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.