Topic: Iosotopes and Avg.
Atomic Mass
 Do
Now:
1)
2)
3)
4)
5)
Dalton
1766
Atoms (tiny)
Atoms of same element
are identical *
Indestructible *
Combine in whole # ratio
(H2O)
In chem rxn. are
combined, separated,
rearranged
*turned out to not be entirely be
true
Remember
#2…well Dalton
was correct…
These three hydrogens are found in nature




All H’s have 1 proton
H-1: mass # = 1
 1-1 = 0 neutrons
H-2: mass # = 2
 2-1 = 1neutron
H-3: mass # = 3
 3-1 = 2 neutrons
Isotopes
 atoms
of same element with
different # neutrons (mass # is
different)
 # protons stays same!
(atomic # is
same)
Consider U-234, U-235, & U-238
the atomic number of U? 92
 How many protons in U? 92
 How many neutrons in U-234? 234 – 92 = 142
 How many neutrons in U-235? 235 – 92 = 143
 How many neutrons in U-238? 238 – 92 = 146
 How many electrons in U? 92
 What’s
Average Atomic Mass
Average Atomic Mass
 The
atomic masses reported in the
periodic table represent the weighted
average of the masses of the
naturally occurring isotopes of that
element
Unit for Avg. Atomic Mass =
 Amu
(or regents says just u) – atomic
mass unit

Define as 1/12 the mass of a Carbon-12
atom
Avg. Atomic Mass of Chlorine
(Cl has 2 isotopes)
% Abundance
Cl-35
Cl-37
75.770%
24.230%
Mass(percent) + mass(percent) +…= avg. atomic mass
35(.75770) + 37(.24230) = 35.453 amu
NOW YOU TRY
Avg. Atomic Mass of Si
92.21% of Si has a mass of 27.97693 amu
4.70% of Si has a mass of 28.97649 amu
3.09% of Si has a mass of 29.97376 amu
Avg. Atomic Mass of Si
0.9221 X 27.97693  25.7975 amu
0.0470 X 28.97649  1.3619 amu
+ 0.0309 X 29.97376  0.9262 amu
28.0856 amu