Chapter 23 Nuclear Chemistry

Origin of the Elements

Chapter 23

Nuclear ChemistryJeffrey MackCalifornia State University,Sacramento1

Images of a human heart before and after stress detecting gamma rays from radioactive Tc-99m Nuclear Chemistry2See Chapter 23 Video Presentation Slide 4Protons(+1) electrical chargemass = 1.672623 1024 gmass = 1.007 atomic mass units (amu)Electronsnegative electrical chargerelative mass = 0.0005 amuNeutronsno electrical chargemass = 1.009 amuAtomic Composition3Isotopes are atoms of the same element (same Z) but different mass numbers (A).Boron-10:5 protons and 5 neutrons: Boron-11:5 protons and 6 neutrons:

10B11B

Isotopes4The isolation and characterization of radium & polonium by Marie Curie was one of milestones of chemistry.It is a credit to her skills as a chemist that she was able to isolate only a single gram of radium from 7 tons of uranium ore.

Marie and Pierre CurieRadioactivity5-particles can be stopped by paper.-particles require at least a cm of lead (Pb).-particles require at least 10 cm of lead (Pb).Energy: > > Natural Radioactivity

6Penetrating Ability

7Ernest Rutherford isolated Radium forms Radon gas while studying alpha particle emission.1902 Rutherford and Soddy proposed radioactivity was the result of the natural change of the isotope of one element into an isotope of a different element.Nuclear Reactions8Alpha emission

Nucleons must be conserved in any nuclear reaction.In emission, the mass number (A) decreases by 4 and the atomic number (Z) decreases by 2.Nuclear Reactions9Beta emissionIn emission, the mass number (A) remains unchanged and the atomic number (Z) decreases by 1.

Nuclear Reactions10Radioactive Decay Series

11Positron (positive electron) emission

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Positrons have the mass of an electron, but positive charge. They are the antimatter analog of an electron.Positron emission arises from electron capture.An inner shell electron is absorbed by the nucleolus converting a proton into a neutron along with an emitted positron.Other Types of Nuclear Reactions12H is most abundant element in the universe.H represents 88.6% of all atomsHe represents 11.3% of all atomsTogether 99.9% of all atom & 99% of mass of the universe.Stability of Nuclei13Hydrogen: 11H, protium21H, deuterium31H, tritium (radioactive)Helium, 42HeLithium, 63Li and 73LiBoron, 105B and 115BIron5426Fe, 5.82% abundant5626Fe, 91.66% abundant5726Fe, 2.19% abundant5826Fe, 0.33% abundantIsotopes14209Bi with 83 protons and 126 neutrons is the heaviest naturally occurring non-radioactive isotope. There are 83 x 126 = 10,458 possible isotopes. Why do so few exist in nature?Stability of Nuclei

15Up to Z = 20 (Ca) stable isotopes often have the same # of neutrons and protons. Only H and He-3 have more protons than neutrons.Beyond Ca, the ratio of neutrons to protons is >1.As Z increases, the n:p ratio deviates further from 1:1Above 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 stable isotopes than ODD Z elements.The more stable isotopes have an EVEN number of neutrons.Stability of Nuclei16 Out of > 300 stable isotopes:EvenOddOddEvenZN157525053115P199F21H, 63Li, 105B, 147N, 18073TaStability of Nuclei

17The trend suggests some PAIRING of NUCLEONSThere are nuclear magic numbers2 He28Ni8 O50Sn20 Ca82 PbEvenOddOddEvenZN15752505Stability of Nuclei

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Band of Stability and Radioactive DecayIsotopes with low n/p ratio, below band of stability decay, decay by positron emission or electron capture

19The energy required to separate the nucleus of an atom into protons and neutrons. For deuterium, 21H21H 11p + 10nEb = 2.15 108 kJ/molEb per nucleon = Eb/2 nucleons= 1.08 108 kJ/mol nucleonsBinding Energy, Eb20For deuterium, 21H: 21H 11p + 10nMass of 21H: = 2.01410 g/molMass of proton: = 1.007825 g/molMass of neutron:= 1.008665 g/molm:= 0.00239 g/molFrom Einsteins equation: Eb = (m)c2 = 2.15 x 108 kJ/molEb per nucleon = Eb/2 nucleons = 1.08 108 kJ/mol nucleonsCalculate Binding Energy21Binding Energy/Nucleon

22The HALF-LIFE of an isotope is the time it takes for 1/2 a sample to decay from its initial amount.The rate of a nuclear transformation depends only on the reactant concentration.The decay and half-life for a nuclear reaction follows first order kinetics.Half-Life23After each successive half-life, one half of the original amount remains.Half-Life

24Activity (A) = Disintegrations/time = (k)(N) where N is the number of atomsDecay follows first order kinetics:

The half-life of radioactive decay ist1/2 = 0.693/k

Kinetics of Radioactive Decay

25Willard Libby (1908-1980)Libby received the 1960 Nobel Prize in chemistry for developing carbon-14 dating techniques. He is shown here with the apparatus he used. Carbon-14 dating is widely used in fields such as anthropology and archeology.Radiocarbon Dating

26Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation:14N + 10n 14C + 1HThe C-14 is oxidized to CO2, which circulates through the biosphere.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 Dating27New elements or new isotopes of known elements are produced by bombarding an atom with subatomic particles such as a protons or neutrons, or even a heavier particles such as 4He and 11B.Reactions using neutrons are called n, reactions because a -ray is usually emitted.Radioisotopes used in medicine are often made by n, reactions.Artificial Nuclear Reactions28An Example of a n, reaction is production of radioactive 32P.32P is used in studies of phosphorous uptake in the body.

Artificial Nuclear Reactions29Elements beyond 92 (transuranium) are made via n, reactions.

Transuranium Elements30106Sg

Transuranium Elements & Glenn Seaborg31Nuclear Fission

32Fission chain reaction has three general steps:Initiation: Reaction of a single atom starts the chain (e.g., 235U + neutron)Propagation:236U fission releases neutrons that initiate other fissionsTermination.Consumption of the fissionable material is completedNuclear Fission33109Mt

Nuclear Fission & Lise Meitner34Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.17% of the worlds energy comes from nuclear.Nuclear Fission & Power

35Curie: 1 Ci = 3.7 1010 distintegrations/s (dps)SI unit is the becquerel: 1 Bq = 1 dpsRad: measures amount of energy absorbed1 rad = 0.01 J absorbed/kg tissueRem: roentgen equivalent man based on amount and type of radiation. Quantifies biological tissue damage, usually represented millirems.Units for Measuring Radiation36Effects of Radiation

37Effects of Radiation

38Nuclear Medicine: Imaging

39Technetium-99m is used in more than 85% of the diagnostic scans done in hospitals each year. Synthesized on-site from Mo-99.

99m43Tc decays to 9943Tc giving off a -ray.The half-life of the radioisotope is 6.01 hrs.Once ingested, the Tc-99m concentrates in areas of high activity such as the thyroid. -ray imagining detects its presence.

Nuclear Medicine: Imaging40Imaging of a heart using Tc-99m before and after exercise.Nuclear Medicine: Imaging

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10B isotope (not 11B) has the ability to capture slow neutronsIn 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 + photonBNCTBoron Neutron Capture Therapy42Food can be irradiated with rays from 60Co or 137Cs.Irradiation retards the growth of bacteria, molds and yeasts.Irradiated milk has a shelf life of 3 mo. without refrigeration.USDA has approved irradiation of meats and eggs.Food Irradiation43