1. 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
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
Atomic mass

2. 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

3. Average vs. Weighted Average
What is the average weight of the 2 cars?
4,000+5,000 2 =4,500
Regular Average
What would happen if we added in extra information?

4. 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

5. 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

6. Atomic Mass of Copper 63 𝑥 .69 + 65 𝑥 .31 =63.62 𝑎𝑚𝑢 69% : Cu-63
Why are the 2 atomic masses different then?

7. 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

8. Practice
Gallium has 2 stable isotopes, and the masses of Gallium-69 (60.11% abundant) and Gallium-71 (39.89% abundant) are amu and amu, respectively. Calculate the average atomic mass of Gallium

9. Practice
Rubidium has 2 isotopes: Rubidium-85 (atomic mass of amu) and Rubidium-87 (atomic mass of amu). The atomic mass of Rubidium reported on the periodic table is 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

10. Practice
Magnesium has 3 stable isotopes. Calculate its average atomic mass, using the information in the chart below.
Isotope
Mass
Abundance
Mg-24
amu
78.99 %
Mg-25
amu
10.00 %
Mg-26
amu
11.01 %