Lectures written by John Kotz Chemistry and Chemical Reactivity 6th Edition John C. Kotz Paul M. Treichel Gabriela C. Weaver CHAPTER 23 Nuclear Chemistry Lectures written by John Kotz © 2006 Brooks/Cole Thomson

Nuclear Chemistry Pictures of human heart before and after stress using gamma rays from radioactive Tc-99m

Why do you care? PET scans Nuclear Power Space travel Smoke Detectors (Am-241) Ionizing Radiation and X-rays Neutron Activation Exposure (pilots, nuclear accidents, Radon) Carbon Dating Nuclear Weapons

Nuclear Radiation The Process of emitting energy in the form of waves or particles. Comes from the Nucleus of the Atom The Neutrons Instability – Binding Energy E=mc2 Non-conservation of Mass

ATOMIC COMPOSITION Protons positive electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units (amu) Electrons negative electrical charge relative mass = 0.0005 amu Neutrons no electrical charge mass = 1.675523 x 10-24 g relative mass = 1.009 amu

Isotopes Atoms of the same element (same Z) but different mass number (A). Boron-10 (10B) has 5 p and 5 n: 105B Boron-11 (11B) has 5 p and 6 n: 115B 10B 11B

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.

Types of Radiation

Penetrating Ability

Nuclear Reactions Alpha emission Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2. Nucleons are rearranged but conserved

Nuclear Reactions Beta emission Note that mass number (A) is unchanged and atomic number (Z) goes up by 1. How does this happen?

Other Types of Nuclear Reactions Positron (0+1b): a positive electron 207 K-capture: the capture of an electron from the first or K shell An electron and proton combine to form a neutron. 0-1e + 11p --> 10n

Radioactive Decay Series

Stability of Nuclei Heaviest naturally occurring non-radioactive isotope is 209Bi with 83 protons and 126 neutrons There are 83 x 126 = 10,458 possible isotopes. Why so few actually exist?

Stability of Nuclei Up to Z = 20 (Ca), n = p (except for 73Li, 115B, 199F) Beyond Ca, n > p (A > 2 Z) Above Bi all isotopes are radioactive. Fission leads to smaller particles, the heavier the nucleus the greater the rate. Above Ca: elements of EVEN Z have more isotopes and most stable isotope has EVEN N.

Stability of Nuclei Even Odd Z N 157 52 50 5 Suggests some PAIRING of NUCLEONS Something inside the nucleus gives each atom a probability of radioactive decay

Band of Stability and Radioactive Decay 24395Am --> 42a + 23993Np a emission reduces Z b emission increases Z 6027Co --> 0-1b + 6028Ni Isotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture

Binding Energy, Eb Eb is the energy required to separate the nucleus of an atom into protons and neutrons. Use E=mc2 Find the mass of the isotope. Sum the masses of the nucleons. For m, use the DIFFERENCE between those masses.

Calculate Binding Energy For deuterium, 21H: 21H ---> 11p + 10n Mass of 21H = 2.01410 g/mol Mass of proton = 1.007825 g/mol Mass of neutron = 1.008665 g/mol ∆m = 0.00239 g/mol = 2.39x10-6 kg/mol c = 3x108 m/sec From Einstein’s equation: Eb = (∆m)c2 = 2.15 x 1011 J/mol How much binding energy is there per nuclear particle? Eb per nucleon = Eb/2 nucleons = 1.08 x 108 kJ/mol nucleons

Half-Life HALF-LIFE is the time it takes for 1/2 a sample to disappear. The rate of a nuclear transformation depends only on the “reactant” concentration. It does not depend on any factors outside the nucleus. Half-life is a property that can be used to identify an element. Half-life cannot predict the likelihood a single atom will decay

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

Kinetics of Radioactive Decay Activity (A) = Disintegrations/time N is the number of atoms Decay is first order, and so ln (A/Ao) = -kt or ln (A) – ln (Ao) = -kt The half-life of radioactive decay is t1/2 = 0.693/k

Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14N + 1on ---> 14C + 1H The C-14 is oxidized to CO2, which circulates through the biosphere. There is a constant % of C-14 in the atmosphere. While a plant is alive, it has the same % of C-14 in it as the atmosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t1/2 = 5730 years. Activity of a sample can be used to date the sample.

Radiocarbon Dating

Man-made Eyes to See Small Things Humans needed to find a way to extend their senses, to gather knowledge about things beyond our physical constraints. Light can be thought of as a piece of information sent between matter. The wavelength/frequency/energy of light determines how it interacts with matter and also predicts where it came from. Certain materials can “see” light that our eyes cannot. Using these materials we learn about the elements in space and on earth.

Human Limitations The molecules in our eyes only work within a very specific range of wavelengths.

Our Sun- Seen by Ultraviolet Light

Extending Our Vision Common detector materials that interact with light: Sodium Iodide crystal: Plastic scintillator: Germanium Crystal: Silicon:

Cosmic Rays Super fast particles from the sun and outer space (protons and ions)--- Strike the atmosphere and become pions (positively charged fundamental particle), then muons (heavy electrons). Built a detector to “see” them using a plastic scintillator.

Cosmic Rays Obtainable info: Direction of radiation Shielding effects Proton from sun Cosmic Rays Molecule in atmosphere Obtainable info: Direction of radiation Shielding effects Pyramids example Depth inside Earth Solar activity levels Pion Muon Neutrino Light Atom of Hydrocarbon 200 muons/m2/second Photomultiplier Tube (PMT)

Cosmic rays are the source of C-14 used in radiocarbon dating!

Terrestrial Radiation Uses gamma ray spectroscopy to “see” light that comes from matter in the ground Obtainable info: Naturally occurring radioactive isotopes can be identified. Composition of isotopes in rocks is compared to rocks from around the world. Background radiation in the air can be measured Investigation of radiation in the ground.

Summary Certain materials interact with the light that our eyes don’t detect. Devices made from these materials have lead to the field of spectroscopy, meaning “seeing light.” All modern devices convert a light signal into an electrical signal. The electrical signal is arranged in a way that allows us to ‘see’ what is going on with our eyes.

Bubble Chambers Alpha, Beta, and Gamma Particles rip through a supercooled gas, ionizing them, and forming bubbles.

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. Radioisotopes used in medicine are often made by these n,g reactions.

Neutron Activation Applications: Shoot neutrons into a substance, stuffing them into a nucleus to make it unstable. They will then decay in a special way that we can “see” what is in them. Applications: Test for the presence of heavily shielded dangerous nuclear material. Create small amounts of elements (alchemy) Find approximate percent compositions of elements in a substance.

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

Transuranium Elements & Glenn Seaborg

Nuclear Fission

Nuclear Fission 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 & Lise Meitner 109Mt

Nuclear Fission & POWER Currently about 104 nuclear power plants in the U.S. and about 400 worldwide. 17% of the world’s energy comes from nuclear fission. What are would be the benefits and drawbacks to using nuclear FUSION instead of nuclear fission?

Nuclear Medicine: Imaging

BNCT Boron Neutron Capture Therapy 10B isotope (not 11B) has the ability to capture slow neutrons In BNCT, tumor cells preferentially take up a boron compound, and subsequent irradiation by slow neutrons kills the cells via the energetic 10B --> 7Li neutron capture reaction (that produces a photon and an alpha particle) 10B + 1n ---> 7Li + 4He + photon

Food Irradiation Food can be irradiated with g rays from 60Co or 137Cs. Irradiated milk has a shelf life of 3 mo. without refrigeration. USDA has approved irradiation of meats and eggs.

Effects of Radiation Rem: Quantifies biological tissue damage Usually use “millirem”