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Chapter2 - 1 ATOMIC STRUCTURE Two components of an atom...

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1 ATOMIC STRUCTURE Two components of an atom Nucleus (pl. nuclei) - composite particle - size 10 -15 m = 10 -4 ° - positively charged - SI units multiples of 1.609 x 10 -19 C - atomic units +1, +2, +3, …, +118 - mass varies from 1.7 x 10 -27 to 5 x 10 -25 kg - 1 amu to 293 amu - contains almost all the mass of an atom - occupies very small volume in the atom Electrons – e - - fundamental particle of nature - size unknown r < 10 -18 m - negatively charged - SI units -1.609 x 10 -19 C - atomic units -1 - mass - SI units 9.109 x 10 -31 kg - atomic units 5.486 x 10 -4 amu - movement of electron occupies the size of the atom 1 ° = 10 -10 m Definition: 1 Angstrom ( ° ) = 10 -10 m NUCLEAR STRUCTURE Two components of nucleus Proton – p + - size 10 -15 m = 10 -4 ° - charge +1 (atomic units) - mass 1.673 x 10 -27 kg = 1.0073 amu - number of p + defines element Neutron – n ° - size 10 -15 m = 10 -4 ° (same as proton) - charge 0 (atomic units) - mass 1.675 x 10 -27 kg = 1.0086 amu - number of n ° defines isotope of element ATOMIC SYMBOLS - Z A Sy Atomic Number - Z - number of protons in an atom - subscript before elemental symbol Mass Number – A - number of protons and neutrons in an atom - superscript before elemental symbol - mass number approximates how heavy the atom is in atomic mass units
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2 Examples Oxygen – 16 has 8 p+ and 8 n ° 8 16 O element # of p+ # of n ° mass # symbol oxygen 8 9 17 8 17 O oxygen 8 10 18 8 18 O carbon 6 6 12 6 12 C carbon 6 8 14 6 14 C lead 82 126 208 82 208 Pb Definition: isotope – an atom with the same atomic number as another atom but a different mass number. - i. e., isotopes have the same number of protons but a different number of neutrons. - isotopes have the same chemical properties. Notes: - oxygen – 16 and oxygen – 18 are isotopes of oxygen - carbon – 12 and carbon – 14 are isotopes of carbon - nuclide refers to specific nucleus, often used interchangeably with isotope DEFINITION OF ATOMIC MASS UNIT Mass of 12 C is 12.000000 amu BY DEFINITION 1 amu = 1.66 x 10 -27 kg amu also abbreviated as u 1 amu = 1 u For atomic masses, amu is much more convenient unit than kilograms m( 1 H) = 1.6737 x 10 -27 kg = 1.0079 amu m( 14 N) = 2.3253 x 10 -26 kg = 14.004 amu m( 56 Fe) = 9.2882 x 10 -26 kg = 55.935 amu
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