1. 4.2 Periodic Table Squares and Average Atomic Mass

2. Element Name Atomic Number Chemical Symbol Atomic Mass
Atomic Number: Number of Protons
Atomic Mass: Average mass based on the abundance of an element’s isotopes
Chemical Symbol
Atomic Mass

3. Mass Number = # of protons + # of neutrons
Learning Check
Based on the atomic number and mass number, find the number of neutrons in an oxygen atom.
(Hint: assume the atomic mass is 16)
Answer: 8 neutrons

4. Isotopes
When the NUMBER OF NEUTRONS changes in an atom.

5. Average Atomic Mass
Represents the average mass of an element based on its isotopes.
UNITS are amu (atomic mass units)
Average Atomic Mass =
(% abundance x mass) + (% abundance x mass) + (% abundance x mass) …..

6. Let’s try this together
Lithium’s isotopes have an abundance ratio of
7.5% of Lithium-6, and 92.5% of Lithium-7. What is
Lithium’s average atomic mass?
Answer:
(% abundance x mass) + (% abundance x mass) = average atomic mass
(0.075 x 6) + (0.925 x 7) = amu

7. Try this on your own
Iron’s isotopes have an abundance ratio of 15% Fe-55, and 85% Fe-56. What is iron’s average atomic mass?
(.15 x 55) + (.85 x 56) = amu

8. One more for good luck
Chlorine’s isotopes have an abundance ratio of % Cl-35 and 24.22% of Cl-37. What is the average atomic mass of Chlorine?
(.7578 x 35) + (.2422 x 37) = amu