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How atoms differ
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Atomic Number
Moseley discovered that each element had a unique charge (+) in its nucleus Number of p+
Atomic Number = # of p+ = # of e-
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Isotopes
All atoms of an element have the same number of: p+
e-
Number of n0 changesIsotopes – the same element with a different
number of n0 but the same number of p+ and e-
Thus, they have a different mass
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Mass number
To identify isotopesNumber after element’s nameEx:
Carbon-14 Potassium-39
Short hand: 39
19K 14
6C Top number is sum of n0 and p+
Bottom number is the atomic number
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Finding the number of n0
n0 = mass # - atomic #
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Mass of individual atoms
p+ = 1.67 x10-24gn0 = 1.67 x10-24ge- = 1/1840
th of 1.67 x10-24g
So small scientist came up with Atomic mass units (amu) Gave c-12 an exact mass of 12.0000 1 amu = 1/12
th the mass of C-12
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Practice
p+ & e- n0 Isotope Symbol
B Calcium-46
C Oxygen-17
D Iron-57
E Zinc-64
F Mercury-204
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Atomic mass
Weighted average of the isotopes of that elementAverage atomic mass = (isotope mass1 x %1) + (isotope
mass2 x %2) + (isotope mass3 x %3)….
Cl 75% Cl-35 25% Cl-37
(35 x 0.75) + (37 x 0.25) = 35.526.25 + 9.25
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Hydrogen H-1 Abundance 99.9851% H-2 Abundance 0.0151%
Helium He-3 abundance 0.0001373% He-4 abundance 99.9998633%
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Application of average atomic mass
The closer an element is to a whole number for its average atomic mass the more likely it is that it’s most abundant isotope has that atomic mass Ex: F has an atomic mass of 18.998amu F-19 (99%)
There are exceptions Br has an atomic mass of 79.904amu Br-79 (51%) & Br-81 (49%) Br-80 doesn’t exist