Unit 15 Nuclear Chemistry

Overview Nuclear Chemistry Radioactive decay Nuclear Stability Isotopes Nuclear force Radioactive decay Alpha, beta, gamma decay Positron emission Electron capture Nuclear Stability Radiometric Dating Half-life Nuclear fusion Nuclear fission Nuclear energy Mass Defect Nuclear binding energy

Nuclear Chemistry Involves the change in the nucleus of an atom Nuclear reactions are everywhere Produce sunlight Create elements (synthetic and natural in stars) Radiation therapy (cancer treatment) Generate electricity Nuclear weapons

World Energy Use

The Nucleus Remember – the nucleus is comprised of the two nucleons (protons and neutrons) Atomic Number – number of protons Mass Number – number of protons and neutrons together It is effectively the mass of the atom

C Nuclear Symbols 12 6 Mass number (p+ + no) Element symbol Atomic number (number of p+)

Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms Example: There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238

Nuclear Force Strong nuclear force Holds protons and neutrons in nucleus very close together Strongest force known

Nuclear Force Nucleus is not stable when atoms experience certain ratios of protons to neutrons Unstable atoms decay and emit radiation Radioactive decay Elements with more than 83 protons (bismuth) are naturally radioactive

Radioactive Decay Radionuclides: Radioactive elements During radioactive decay The makeup of the nucleus changes The number of protons may change Means that the element has changed

Natural Radioactive Isotopes Radon-222 Comes from decomposition of Uranium rocks 2nd leading cause of lung cancer Comes up through cracks in basements Radium-226 Some radium salts glow in the dark Early 1900s used to be used as paint for watches and clocks (workers licked paint brushes and got cancer – “radium girls”) Uranium-238 Rocks create radon gas Used in radioactive dating Potassium-40 One of few light radioactive elements Produces argon that is found in atmosphere

Other Common Radioisotopes Use 14C Archaeological dating 24Na Circulatory system testing for obstruction 32P Cancer detection 51Cr Determination of blood volume 59Fe Measurements of red blood cell formation and lifetimes 60Co Cancer treatment 131I Measurement of thyroid activity 153Gd Measurement of bone density 226Ra 3H 235U Nuclear reactors and weapons 238U 241Am Smoke detectors

Measuring Radioactivity One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Radioactive Decay (3 Most Common Types) Alpha (a, He) 2 protons, 2 neutrons Beta (b, e) High energy electron Gamma (g) Electromagnetic radiation High energy photons

Alpha, Beta, Gamma Radiation

He or  U Th He Alpha Decay: + Loss of an -particle (a helium nucleus) He or  4 2 4 2 U 238 92  Th 234 90 He 4 2 +

 e I Xe e Beta Emission: + Loss of a -particle (a high energy electron)  −1 e or I 131 53 Xe 54  + e −1

 Gamma Emission: Loss of a -ray High-energy radiation that almost always accompanies the loss of a nuclear particle Not usually written in nuclear equation 

 e C B e Positron Emission: + Loss of a positron (a particle that has the same mass as but opposite charge of an electron)  1 e or C 11 6  B 5 + e 1 Has a very short life because it is destroyed when it collides with an electron, producing gamma rays: e + e   1 -1

p n e Positron Emission + A positron can convert a proton to a neutron 1  n + e

Electron Capture Capture by the nucleus of an electron from the electron cloud surrounding the nucleus Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron p 1 + e −1  n

Nuclear Stability Several factors predict whether a particular nucleus is radioactive Neutron-to-proton ratio Radioactive series Magic Numbers Evens and Odds

Neutron-Proton Ratios The strong nuclear force helps keep the nucleus from flying apart Protons repel each other Neutrons help the strength of the nuclear force As protons increase, neutrons have to counter-act increasing proton-proton repulsions In low atomic number elements (1-20) protons and neutrons are approximately equal In high atomic number elements number of neutrons much larger than protons Neutron-proton ratio helps stabilize nucleus

Neutron-Proton Ratios For smaller nuclei (Atomic Number  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. (If an isotopes mass number is greater than its atomic weight, the same trend will happen example C) 16 6

Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture. (If an isotopes mass number is less than its atomic weight, the same trend will happen example C) 11 6

Stable Nuclei There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission. Decreases both protons and neutrons

Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). Often occur in nature

Magic Numbers Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons. These are called the “Magic Numbers”

Evens and Odds Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

Kinetics of Radioactive Decay Radioactive decay is a 1st order process Remember this equation: = t1/2 0.693 k

Radiometric Dating Half life can help determine the age of different objects Carbon-14 Half life of 5,715 years Can determine age of organic materials up to about 50,000 years old

Radiometric Dating Uranium-238 Half life of 4.5×109 years Used to determine age of Earth (measured rocks) Oldest rock found is almost 4.5 billion years old

Nuclear Fusion Elements can be man-made by bombarding nuclei with particles Alpha particles accelerated and collided with nucleus Neutrons bombard nucleus Bombard nuclei to create transuranium elements Heavy elements beyond uranium on periodic table

Particle Accelerators Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide These particle accelerators are enormous, having circular tracks with radii that are miles long

Nuclear Fission The splitting of heavy nuclei (Fusion is the combination of light nuclei) Process begins by bombarding heavy nucleus with a neutron 2 main commercial uses Nuclear Weaponry Nuclear Energy

Nuclear Fission This is called a chain reaction About 2 neutrons are produced for each fission These 2 neutrons cause 2 additional fissions Which cause 2 more fissions each Which cause 2 more fissions each… This is called a chain reaction

Nuclear Fission Chain reactions can escalate quickly If the reaction is not controlled, it results in a violent explosion because of the release of too much energy too quickly

Nuclear Energy We can control fission reactions and use it to create energy

Nuclear Energy Fission reactions are carried out in nuclear reactors The reaction is kept in check by the use of control rods These block the paths of some neutrons, keeping the system from escalating out of control The heat generated by the reaction is used to produce steam that turns a turbine connected to a generator Video: http://www.youtube.com/watch?v=VJfIbBDR3e8

Debates on Nuclear Energy Pros… Cleaner energy than coal and fossil fuel plants Doesn’t add to global warming High amount of electricity can be generated in one plant Cheaper to run a nuclear facility than a fossil fuel plant Cons… Nonrenewable source of energy Produces nuclear waste that must be stored for thousands of years Accidents (Chernobyl, Three Mile Island, Fukushima) http://www.youtube.com/watch?v=eGI7VymjSho Very expensive to build a nuclear facility (about $10 billion per reactor)

Nuclear Energy E = mc2 We can measure the energy associated with nuclear reactions E = mc2 E = energy (J) m = change in mass (kg) during reaction (mass of products-mass of reactants) c = speed of light (3.0×108 m/s) When a system loses mass, it is exothermic (-E) When a system gains mass, it is endothermic (+E)

Nuclear Energy The mass change in chemical reactions is so small that we treat them as though mass is conserved Ex: Mass change for exothermic process of combustion of 1 mol of CH4 is -9.9×10-9 grams Mass change in nuclear reactions is measureable Ex: Mass change accompanying decay of 1 mol of uranium-238 is 50,000 times greater than combustion of CH4

Nuclear Energy (example) For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g. The change in energy, E, is then E = (m) c2 E = (−4.6  10−6 kg)(3.00  108 m/s)2 E = −4.1  1011 J

Mass Defect When protons and neutrons form a nucleus, the mass of the nucleus is less than the sum of the masses of its constituent protons and neutrons Example: Helium (He) – 2 protons, 2 neutrons Protons and Neutrons Mass of Nucleus Mass of 2 protons (2×1.0073 = 2.0146) 4.0015 amu Mass of 2 neutrons (2×1.0087 = 2.0174) Total mass = 4.0320 amu Difference = 4.0320 – 4.0015 = 0.0305 amu (mass defect)

Mass Defect E = mc2 E = (5.1×10-29 kg)(3.0×108 m/s)2 E = 4.6×10-12 J To measure the energy associated with the mass defect use E = mc2 Example: Helium (He) – 2 protons, 2 neutrons E = (5.1×10-29 kg)(3.0×108 m/s)2 E = 4.6×10-12 J NOTE: 1 gram = 6.022×1023 amu

Nuclear Binding Energy Energy required to separate a nucleus into its individual nucleons (protons and neutrons) Also use E = mc2 The larger the binding energy, the more stable the nucleus toward decomposition