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 20.18 amu?
Calculated as a weighted average of atoms in
a sample of naturally occurring isotopes.
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?
copper-63
has a mass of 62.93 amu
and natural abundance of 69.15%
copper-65
has a mass of 64.93 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
neon-20
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
20.18 amu.
Quick Quiz!
1. If an unusually selected sample of sulfur
contained 90.0% sulfur-32 and
10.0% sulfur-34, what would be its
average atomic mass?
A. 32.2 amu
B. 32.9 amu
C. 33.5 amu
D. 34.2 amu
Avg. = (M1)(%) + (M2)(%)
= (32)(0.900) + (34)(0.100)
= 32.2