1. Average Atomic Mass If there are 2 naturally occurring isotopes of
Neon including
Ne-20 and Ne-22.
then the average mass of Neon atoms should be ___ amu.
Why does the PT lists it as amu?
Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

2. 63Cu is 69% abundant 65Cu is 31% abundant Average Atomic Mass
A weighted average atomic mass is calculated from both the
masses of each isotope and the
relative abundance (%) of each isotope.
63Cu is 69% abundant
65Cu is 31% abundant
To which will the
average mass be closer,
63 or 65?

3. = 63.55 Calculating Average Atomic Mass
copper-63
has a mass of amu
and natural abundance of 69.15%
copper-65
has a mass of amu
and has a natural abundance of 30.85%
Calculating Average Atomic Mass
Avg. Mass = (Mass1)(%) + (Mass2)(%) …
= (62.93)(0.6915) + (64.93)(0.3085)
= 63.55

4. neon neon-22
Which isotope is in greater relative abundance in natural samples of neon?
because…
the weighted average atomic mass of neon from the PT is amu.
90% - 20, <1% - 21, 9% - 22

5. = 32.2 Avg. = (M1)(%) + (M2)(%) = (32)(0.900) + (34)(0.100)
Quick Quiz!
1. If an unusually selected sample of sulfur contained 90.0% sulfur-32 and % sulfur-34, what would be its average atomic mass?
32.2 amu
32.9 amu
33.5 amu
34.2 amu
Avg. = (M1)(%) + (M2)(%)
= (32)(0.900) + (34)(0.100)
= 32.2