1. Unit 2: Atomic Theory & Structure
Average Atomic Mass

2. Mass of Atoms
One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.
 0
 1
 1

3. 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.

4. 10B is 19.8% abundant 11B is 80.2% 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.
10B is 19.8% abundant
11B is 80.2% abundant
To which will the
average mass be closer,
10 or 11?

5. = 10.81 amu Calculating Average Atomic Mass
boron-10
has a mass of amu
and natural abundance of 19.8%
boron-11
has a mass of amu
and has a natural abundance of 80.2%
Calculating Average Atomic Mass
Avg. Mass = (Mass1)(%) + (Mass2)(%) …
= (10.01)(0.198) + (11.01)(0.802)
= amu

6. Calculating Average Atomic Mass
Element X has two natural isotopes:
X-20 (mass of amu); %.
X-22 (mass of amu); %.
Calculate the average atomic mass.
20X: (20.012)(0.3991) =
7.987
22X: (22.009)(0.6009) =
13.225
amu

7. = 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