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Lecture

# Isotopic abundance, calculating atomic mass, modern atomic theory vs. Dalton's, periodic table,

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Department
Chemistry
Course
CHEM 111
Professor
Dana Johnson
Semester
Fall

Description
1 September Isotopic abundance Isotopes – atoms of an element with a different # of neutrons, can vary the mass of the elements, most are chemically equivalent, exist in different amounts For Carbon 12C (carbon – 12) most abundant, 6 n , 98.89% 13C (carbon – 13) 7 n , 1.11% 14 0 C (carbon – 14) 8 n , <0.01% Affects weight on periodic table Reported weight is an average of all isotopes, taking into account their abundance “Atomic mass” or “atomic weight” Used for calculations Calculating atomic mass Atomic mass unit (amu) – standard of measure for all atomic masses. 1/12 the mass of the carbon – 12 atom, also called the Dalton (Da) Total weight = Σ portions of each isotope Portion of each isotope = mass of isotope x % abundance Clicker question Copper has 2 naturally occurring isotopes 63 C with a mass of 62.9396 amu 65C with a mass of 64.2978 amu If copper has an atomic weight of 63.546 amu, what is the percent abundance of each isotope? Clicker solution The abundances of the 2 isotopes must equal 100%. So, set one equal to x and the other to 1 – x. 62.9396(x) + 64.2978(1 – x) = 63.546 62.9396x + 64.2978 - 64.2978x = 63.546 62.9396x - 64.2978x = 63.546 - 64.2978 -1.9882x = -1.3818 (only 4 sig figs though) X = -1.3818 / -1.9882 X = 0.695001 → 0.6950 (4 sig figs) → 69.50% X – 1 = 0.3050 → 30.50% Modern atomic theory vs. Dalton’s #1: All matter is composed of atoms, the smallest unit to retain the characteristics of the element, divisible into sma
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