Name: ____________________________________________ Period: _____ Date: ____________________
Calculating Average Atomic Mass
What is an isotope? _________________________________________________________________________
There are three isotopes of potassium that occur naturally:
K – 39 has 20 neutrons
39
19
K
K – 40 has 21 neutrons
40
19
K
K – 41 has 22 neutrons
41
19
K
Chemists have developed a method of measuring the mass of an atom called: ___________________________

abbreviated “___________”

They did this because the masses of subatomic particles are ____________________________that even
scientific notation makes it hard to calculate atomic masses of elements!
The atomic mass that you see on the periodic table is the __________________________________ of the
isotopes of that element.
*The atomic mass you see on the periodic table is also the isotope that ________________________________
in nature!
Question: There are three isotopes of potassium: K-39, K-40, and K-41. Which isotope is more abundant in
nature?
Answer: ______________
Question: Chlorine exists naturally as chlorine – 35 and chlorine – 37. The atomic mass of chlorine is 35.453
amu. Which isotope is more abundant in nature?
Answer: ______________
Practice Problems
**You can calculate the average atomic mass you see on the periodic table for any element!
You need to know:
1. the masses of the isotopes
2. their percent abundance
avg. atomic mass = mass (in amu) x percent abundance in decimal form
1. Magnesium has 3 isotopes. Calculate the atomic mass of magnesium if one isotope has a mass of 23.985
amu and a relative abundance of 78.99%, a second isotope has a mass of 24.986 amu with a relative
abundance of 10.00%, and a third isotope has a mass of 25.982 amu with a relative abundance of 11.01%.
2. Find the average atomic mass of boron if 19.8% of the boron occurring in nature has a mass of 10.013 amu
and 80.2% of boron atoms found in nature have a mass of
11.009 amu.
3. An element has two naturally occurring isotopes. Ag-107 has an abundance of 51.82% and a mass of 106.9
amu. Ag-109 has a relative abundance of 48.18% and a mass of 108.9 amu. Calculate the average atomic
mass of silver.
4. Helium has two naturally occurring isotopes: helium-3 and helium-4. The atomic mass of helium is 4.003
amu. Which isotope is more abundant in nature? Explain why.