Chapter-19-Tro-4-19-09

Chapter-19-Tro-4-19-09 - Radioactivity and Nuclear...

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Radioactivity and Nuclear chemistry Nuclide : an atom whose composition of nucleus is known Nucleus : a tiny portion of atom core where protons and neutrons reside. Z : atomic number (number of protons in the nucleus of an atom) N : number of neutrons in the nucleus of an atom. A =Z+N= atomic mass number is integer closest to the relative atomic mass : X A z Left superscript A is atomic mass number; subscript Z is the atomic number (1). The neutrons provide short range attractive interactions that help contain the protons which have dominant electrostatic repulsive interactions. (2). For up to 20 protons, neutrons are needed at 1:1 ratio for stability. For lighter elements, stable nuclei have the ratio of neutrons to protons (N/Z) ~1 For Carbon-12; Z=6; N=6; N/Z=1 Beyond Z=20, the (N/Z) ratio for stable nuclei increases. For Zirconium (Zr), Z=40, N=50 and (N/Z) ~1.25; For mercury (Hg), Z=80, N=120 and (N/Z) ~1.5 (3). Holding more than 83 protons in the same nucleus is not possible, no matter how many neutrons are present. Bismuth (Z=83) is the heaviest element with stable nucleus (4). Although neutrons lend stability to protons, neutrons are not stable as free entities. (5). A neutron decays with a half-life of 14.6 minutes into a proton and an electron. When excess neutrons are present in a nucleus they can decay as follows:
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- - + + = e p n 0 1 1 1 1 0 Here left superscript gives the mass number; left subscript gives the charge Proton with mass number of 1 and charge of +1 electron with mass number of 0 and charge of -1 neutron with mass number of 1 and charge of 0 (6). In the neutron decay reaction, : (a). Mass number is conserved; (b). Charge is conserved; (c). But the mass is not conserved. We will use Atomic mass unit (amu) for masses . One Avogadro’s number of atomic mass units is equal to 1 g. Atomic mass unit is represented by symbol “u” or as “amu”. g u 1 10 0221420 . 6 23 = × kg g g u 27 24 10 6605387 . 1 10 6605387 . 1 10 0221420 . 6 1 23 - - × = × = × = (7). m = mass of proton + mass of electron - mass of neutron =1.0072765u + 0.0005486 u - 1.0086649u = -0.0008398 u = -0.0008398 u x (1g/6.022142x10 23 u)= - 1.395x10 -27 g = - 1.395x10 -30 kg (8). In a neutron decay the mass decreases, but the decreased mass is related to change in energy as, E= m. c , where c is the velocity of light in vacuum. This
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(12). This energy change in decay of 1 mol neutrons is thousands of times larger than that in chemical reactions. (13). Energy changes in nuclear reactions are expressed in electron volts (ev). 1 electron volt = energy given to an electron when the electron is accelerated through a potential difference of 1 V. E= charge in Coulombs (Q) x potential in volts ( E ) . =1.602176x10 -19 C x 1 V= 1.602176x10 -19 C.V= 1.602176x10 -19 J (14). One megaelectron volt, MeV= 10 6 eV= 1.602176x10 -13 J (15). A convenient unit conversion is energy change per 1 u. u MeV J MeV u J s m u kg c m E 494 . 931 10 602176 . 1 1 ) 10 492418 . 1 ( ) . 10 997925 . 2 )( 10 660539 . 1 ( . 13 10 2 1 8 27 2 = × × = × × = = - - - - (16). Energy equivalent of 1 u change = 931.494 MeV.
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This note was uploaded on 03/06/2010 for the course CHEM 102B taught by Professor A during the Spring '09 term at Vanderbilt.

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Chapter-19-Tro-4-19-09 - Radioactivity and Nuclear...

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