What is atomic mass?
Also referred to as:
 Atomic weight
 Average atomic mass
 Relative atomic mass
Important characteristic for elements:
 Each element has its own atomic mass
Atomic mass is an average
Atomic mass
• Average of the masses of a number of
different atoms
• Special kind of average called a weighted
average
• Different than the usual average you’re
probably familiar with in math
Understanding Weighted Averages
Even though these are different
models and have different
features, they are both lemonas
due to their distinct lemon-like
shape
Using this analogy, the models of the lemona are similar to the isotopes of an element
• 29 protons makes both atoms
copper—even though they
differ in their numbers of
neutrons
• Just like the “lemon-like” shape
of a car makes it a lemona—
even though they differ in their
features
Average vs. Weighted Average
What is the average weight of the 2
cars?
4,000+5,000
•
= 4,500
2
• Regular Average
What would happen if we added in extra information?
Weighted Average
What is the average weight of lemonas, taking
into account the amount of each model?
4,000 𝑥 .95 + (5,000 𝑥 .05)
= 4,050 𝑙𝑏𝑠
Because there are so many more GXs than GXLs, the weighted
average is much closer to the actual weight of a lemona GX
Using Weighted Average with Different
Atoms
• Atomic mass: a weighted average of the masses for
all the isotopes of a certain element
Mass = 63 amu
Mass = 65 amu
If we pulled out a random sample of 100 copper atoms, we would find that 69% of
them would be Cu-63 and 31% of them would be Cu-65
• 69% : Cu-63
• 31% : Cu-65
Atomic Mass of Copper
69% : Cu-63
31% : Cu-65
63 𝑥 .69 + 65 𝑥 .31 = 63.62 𝑎𝑚𝑢
Why are the 2 atomic masses different then?
Difference between mass number and
atomic mass
Mass number: protons + neutrons
1 proton or 1 neutron = 1 amu
• Therefore, if you have 6 protons and 6
neutrons, your atom is going to weigh 12
amu
• If you have 6 protons and 7 neutrons, your
atom is going to weight 13 amu
Practice
Gallium has 2 stable isotopes, and the masses of Gallium-69
(60.11% abundant) and Gallium-71 (39.89% abundant) are
68.926 amu and 70.925 amu, respectively. Calculate the average
atomic mass of Gallium
Practice
Rubidium has 2 isotopes: Rubidium-85 (atomic mass of 84.911
amu) and Rubidium-87 (atomic mass of 86.909 amu). The
atomic mass of Rubidium reported on the periodic table is 85.47
amu. Based on this information, which of the isotopes of
Rubidium is more abundant? How do you know?
•
This is more of a thought problem. No real calculations are really necessary
Practice
Magnesium has 3 stable isotopes. Calculate its average
atomic mass, using the information in the chart below.
Isotope
Mass
Abundance
Mg-24
23.985 amu
78.99 %
Mg-25
24.986 amu
10.00 %
Mg-26
25.983 amu
11.01 %