Nuclear Chemistry Reactions and properties of nucleus M p n Mass number (number of nucleons) nuclide : nuclear species Isotopes : nuclides of same chemical species Nuclear equation Atomic number (number of protons)

Nuclear Chemistry Selected elementary particles (atomic) mass unit : 1/12 the mass of a single atom of 12 C same as “dalton” 1u(amu) = 1 dalton = 1.66 x Kg

Nuclear Chemistry Einstein mass-energy relationship balanced nuclear equation, but  m = -8.4 x u Change in mass corresponds to change in energy, and vice versa. i.e.  E =  m C 2 = x J = MeV 1 eV = 1.60 x J Energy obtained when e was accelerated by 1V potential difference. cf. In chemical reaction,  m ~ 0 Ex. CH 4 의 연소열  H= -890KJ  m = -9.9 x g In nuclear reaction  m = g

Nuclear Chemistry Nuclear binding energy mass of nucleus < mass of nucleons nuclear binding energy i.e.  E =  m C 2 : nuclear binding energy stability of nucleus Nuclear decay process : spontaneous i.e.  G < 0  m = mass defect 1. Beta decay Atomic number : +1, mass number : unchanged # of nucleons ~  m < 0 Neutrino

Nuclear Chemistry 2. Positron emission Atomic number : -1, mass number : unchanged 3. electron capture When positron emission cannot occur 4. Alpha decay Atomic number : -2, mass number : othersNeutron emission, proton emission, fission All toward more stable nucleus.

Nuclear Chemistry 1. Photographic plates(emulsion)darkening is proportional to radiation 2. Scintillation counter Emits light when absorbs beta particle or gamma ray : phosphorescence can count number of light pulses. 3. Geiger counter Beta particle produces positive ions and electrons in a tube filled with gas. Then electric current is generated. Detecting radioactivity

Nuclear Chemistry is first order rate kinetics then, N t = N o e -kt Kinetics of radioactive decay half life t 1/2 = Measured radioactivity (A) is the decay rate = kN Radioactivity unit : becquerel (Bq) – 1 radioactive disintegration per second curie (Ci) – activity of 1g of radium is 1 Ci 1 Ci = 3.7 x Bq

Nuclear Chemistry If we know half-life of a radioactive nuclide Radioactive dating 1. for the age of uranium containing rock We should know A o to find out the age of the sample. Assumption : Pb in the rock came only from uranium decay. Then N t (Pb) = N o (U) – N t (U) = Nt(U) (e kt – 1)Where N t = N o e -kt t =

Nuclear Chemistry 2. Radiocarbon dating : 14 C decay Assumption : 14 C/ 12 C ratio is constant during last 50,000 years. This can change with burning of fossil fuel that increases 12 C in the atm. Produces 14 C all the time and therefore In nature From this, we can deduce A o of carbon source from fossils or dead ones. And the time of its death. This is the ratio in CO 2 in the atm and thus is the raio in the living organisms !

Nuclear Chemistry 3. Radiation in biology and medicine Damage is proportional to the quantity and energy of the particle Somatic damage – burning, cancer Genetic damage – DNA mutation Safe level of exposure : ~ 100 mrem/yr. (from nature), ~ 100 mrem/yr. (from human activity) LD 50 (in 30 days) = 500 rad 1 Gy(gray) = 100 rad, 1 Sv(sievert) = 100 rem Effect of radiation : chemical changes in cellular molecules leads to change in cellular function and becomes cancerous Measurement of radiation damage 1 rad of beta or gamma radiation = 1 rem, 1 rad of alpha radiation = 10 rem rad ( radiation absorbed dose) : the amount of radiation that deposits 0.01J/Kg rem ( roentgen equivalent in man) : effective dosages of radiation received by humans RBE : relative biological effectiveness rem = rad x RBE

Nuclear Chemistry Radon : gaseous, generated from rocks and soils, especially from granite 222 Rn decay quickly but its products are also radio active and can accumulate in the confined air.

Biology : biosynthetic route of biomolecules Medical use of radiation : X-ray – imaging, cancer therapy 131 I – thyroid cancer PET(positron emission tomography) : diagnostic tool for cancer, neurological disease good tool to study brain function T 1/2 = 20.3 min. T 1/2 = min. Normal Brain Image of the brain of a 9 year old female with a history of seizures poorly controlled by medication. PET imaging identifies the area (indicated by the arrows) of the brain responsible for the seizures. Through surgical removal of this area of the brain, the patient is rendered "seizure-free".

Nuclear Chemistry Fragmentation of an element by neutron bombardment Nuclear Fission

Nuclear Chemistry Fragmentation of an element by neutron bombardment Nuclear Fission Self-propagating chain reaction Controllable by absorbing produced neutrons( 112 Cd or 10 B) 235 U, 233 U, 239 Pu can be used. Nuclear power reactor Fuel : 235 U (~3% for “light water” reactor, 0.7~1.5% for “heavy water” reactor) Moderator : slows down neutron and transfer heat to a steam generator: water, carbon Controller: controls fission reaction rate by capturing neutrons

Thermonuclear reactions : requires very high kinetic energies Nuclear Fusion Requires 40,000,000K Hydrogen burning Helium burning starts at 10 8 K Nucleosynthesis : nuclear transmutation In stars Fusion reactor : requires very high temperature and control --- still a dream cf. cold fusion

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