Number of Protons = Number of Electrons
Number of Protons = Atomic Number
Atomic Mass = Protons + Neutrons
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
Weighted Avg. of Test Grades
 Suppose


you have:
1 test of 0% and 9 tests of 90%
What if I said your average was 45?
(0 + 90 = 90; Divide by 2 = 45)
 Would
you be happy?
NO!
 The
long way to do this problem:
 (90
+ 90 + 90 + 90 + 90 + 90 + 90 + 90 +
90 + 0)  10
 Average

= 81%
I bet you like this a lot better!
The smart way to do this problem!
 grades
of 90: 9 out of 10 is 90%
 grade 0f 0: 1 out of 10 is 10%
 So
90% of the test grades are 90
 10% of the test grades are 0
 (0.90

X 90) + (0.10 X 0) =
81 + 0
Average = 81%
Weighted average:
Smarter way to calculate
the average if you have
numerous items to average
Method of Weighted
Averages
●
●
●
Convert % to decimal format
(Divide % by 100)
Multiply each isotope’s abundance
factor by its atomic mass
Find the Sum
Avg. Atomic Mass of Chlorine
Chlorine has two isotopes.
Percent
Abundance
Mass
Isotope 1
75.770%
34.969 amu
Isotope 2
24.230%
36.966 amu
Avg. Atomic Mass of Chlorine =
(.75770)(34.969 amu) +
Convert % to
decimals
(.24230) (36.966 amu)
= 26.496 amu + 8.957 amu
= 35.453 amu
Multiply by
appropriate
mass
Sum
Avg. Atomic Mass of Chlorine
 To
estimate the answer:
 75% of Cl is 34.969 amu &
 25% of Cl is 36.966 amu
 The
final answer has to be between
34.969 amu & 36.966 amu, but
closer to 34.969 amu
35.453 amu
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
Avg. Atomic Mass of Pb
1.5% Pb-204  0.015 X 204amu: 3.06amu
23.6% Pb-206  0.236 X 206amu: 48.62amu
22.6% Pb-207  0.226 X 207amu: 46.78amu
52.3% Pb-208  0.523 X 208amu:108.78amu
Sum
207.24amu