Nuclear Chemistry Chapter 21

Nuclear Reactions Some nuclei are unstable. They are said to be radioactive and emit particles and usually high-energy electromagnetic radiation (gamma rays) at the same time. Other nuclear reactions involve bombarding nuclei with particles such as neutrons, protons, alpha particles or other nuclei. (particle accelerators)

Homework problems pages 698-701 1, 3-11, 14-18, 20, 23-25, 28, 29, 33, 34, 36-46, 51, 52, 60, 64, 66, 68, 71

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

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

Example 21.1 Write balanced nuclear equations for the Decay of U-238 to Th-234 Decay of Cs-137 to Ba-137 Emission of a beta particle by As-78

positron decay or electron capture n/p too large beta decay X Y n/p too small positron decay or electron capture 23.2

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

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 Δ E = (Δ m)c2 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

Ex 21. 2 The atomic mass of I-127 is 126. 9004 amu Ex 21.2 The atomic mass of I-127 is 126.9004 amu. Calculate the nuclear binding energy and the BE per nucleon. c = 3.00 x 108m/s 1 kg = 6.02 x 1026 amu 1 J = 1 kg m2/s2

Natural Radioactivity Nuclei outside the belt of stability or with more than 83 protons are unstable and decay naturally. This decay obeys 1st order kinetics and therefore has a constant half-life.

Kinetics of Radioactive Decay N daughter rate = - DN Dt rate = lN DN Dt = lN - N = N0exp(-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 ln2 = t½ l 23.3

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

Final Exam – June 17 EH Assignment Due June 15 Unit 23 Mininum score Sec 1 Properties of Radiation 90 Sec 2 Balancing Nuclear Reactions Sec 3 Predicting Nuclear Stability Sec 4 Radiation Decay Kinetics Sec 5 Radiation Measurement Sec 6 Nuclear Binding Energy Final Exam – June 17

Radiocarbon Dating 14N + 1n 14C + 1H 14C 14N + 0b + n t½ = 5730 years 6 14C 14N + 0b + n 6 7 -1 t½ = 5730 years The C-14 isotope is produced in the upper atmosphere N-14 through the action of cosmic rays. All living organisms contain C-14, but when the organism dies the amount of C-14 decreases. Paper from the Dead Sea Scrolls was found to have only 0.795 times as much C-14 as that of a living plant. What is the age of the scroll? 23.3

K-40 decays to Ar-40 with a half life of 1.2 x 109 years. Age of Rocks K-40 decays to Ar-40 with a half life of 1.2 x 109 years. Uranium-238 Dating 238U 206Pb + 8 4a + 6 0b 92 -1 82 2 t½ = 4.51 x 109 years 23.3

Nuclear Transmutations In 1919 Rutherford bombarded N with alpha particles and produced O-17. This was the first manmade nuclear reaction. Why not just mix N and He? Example 21.3 Neutrons are particularly useful in producing synthetic isotopes since they have no charge.

Nuclear Transmutation Cyclotron Particle Accelerator 14N + 4a 17O + 1p 7 2 8 1 27Al + 4a 30P + 1n 13 2 15 14N + 1p 11C + 4a 7 1 6 2 23.4

Nuclear Transmutation 23.4

A new type of nuclear reaction – fission- was discovered in 1938 A new type of nuclear reaction – fission- was discovered in 1938. The experiments were carried out in Germany by Hahn and Strassman and interpreted by Lisa Meitner. U-235 and Pu-239 will undergo fission when bombarded with neutrons. U-235 makes up 0.7% of natural uranium and Pu-239 can be produced from the more abundant U-238.

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

Representative fission reaction Nuclear Fission Representative fission reaction 235U + 1n 90Sr + 143Xe + 31n + Energy 92 54 38 23.5

Notice that the fission products are not the normal isotopes and are always highly radioactive. This is true of fission bombs and nuclear reactors.

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

The little boy bomb. It used U-235

The fat man bomb. It used Pu-239

Fig. 21.9

Schematic diagram of a nuclear fission reactor Much of the heat comes from decay of fission products. Even if fission stops, this decay does not. 23.5

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

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 High T required. Why? Tokamak magnetic plasma confinement 23.6

Thermonuclear bombs Mark 17 model

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 23.6

Geiger-Müller Counter 23.6

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 23.6

The Standard Model is the name give to the current theory of fundamental particles and how they interact.

There are four fundamental forces in nature. Electromagnetic force Gravity Strong nuclear force Weak nuclear force Electroweak Theory has shown that 1 and 4 are really different aspects of the same force.

There are 12 fundamental fermion particles in nature There are 12 fundamental fermion particles in nature. (spin =1/2, 3/2, 5/2, … 6 flavors of quarks and 6 flavors of leptons. All the matter that we are familiar with are made up of two quarks – up and down and one lepton – the electron.

The other category of particles are the bosons (spin =0, 1, 2,…) The bosons are force carriers. For example, the electromagnetic force is caused by an exchange of photons. Each force has one or more force carriers.

The strong nuclear force is carried by gluons The strong nuclear force is carried by gluons. Quarks come in three colors – red, green and blue. This is analogous to + and – for the electromagnetic force. Unlike colors attract. This force is also called the color force.

Fermilab Ring is 4 miles around. Protons and antiprotons collide.

Fermilab – particle tracks in detector. Discovery of top quark.

Practice Test #5 - 1999

and Ca-45, c. Mo-95, Tc-92 d. Hg-195, Hg-196