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CHEM 1161 (2)
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Boston College
CHEM 1161

3.4 Atomic Number and Mass Number Thursday, September 13, 2012 10:22 AM Electron cloud nucleus electron Atomic Number •Is specific for each element •Same for all atoms of an element •Equal to number of protons in an atom •Appears above the symbol of an element in the periodic table •Atomic number of C is 6,every C atom has 6 protons Neutral Atoms •Net charge is zero •Number of protons=number of electrons • Mass Number •Represents the number of particles in the nucleus •# of protons +# of neutrons •Always a whole number • Does not appear in periodic table •Number of neurons = mass number - atomic number 3.5 Isotopes •Are atoms of the same element •Have different mass numbers •Have same number of protons but different numbers of neutrons Atomic Mass •Weighed average of all the naturally occurring isotopes of that element •Number on the periodic table below the chemical symbol •To calculate: take percent abundance x mass of isotope • EX: gallium there is 60.10% of 69Ga (atomic mass 68.926) atoms and 39.90% of 71 Ga (atomic mass 70.925) atoms What is the atomic mass of Ga? 68.926(60.10/100) =41.42 amu 70.925(39.90/100)=28.30 Add = 69.72 amu Electromagnetic Spectrum • Shows the arrangement of wavelengths of electromagnetic radiation within the visible range from 700 to 400 nm • E=hv E= hc/ h= constant Plank's constant v=frequency of wave c=speed of light lamda=wavlength • Elements have unique atomic spectrum Energy Levels • Are assigned numbers n=1,2,3,4 • Increase in energy as the value of n increases •
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