Finding Atomic Mass
Atomic mass is the weighted average of the masses of all the naturally occurring isotopes
of an element.
To calculate the atomic mass of an element, multiply the mass number of each isotope by
its percentage of abundance in decimal form. Then add these amounts together to find
the atomic mass.
Example: Chlorine-35 makes up 76 percent of all chlorine in nature, and chlorine-37
makes up the other 24 percent. The atomic mass of chlorine is calculated as follows:
35.5 amu
1. 7.5% of all the lithium in the world is Lithium-6 and 92.5% of all the lithium in the
world is Lithium-7. What is the atomic mass of lithium?
2. Nitrogen has two isotopes. 99.6% of all nitrogen is Nitrogen-14. The remaining
0.366% is Nitrogen-15. What is the atomic mass of nitrogen?
3. There are three isotopes of magnesium. The abundance of Magnesium-24 is 79%.
The abundance of Magnesium-25 is 10% and the abundance of Magnesium-26 is
11%. What is the atomic mass of magnesium?
4. Silicon has three isotopes. Silicon-28 makes up 92.2% of all silicon. Silicon-29
makes up 4.7% of all silicon and Silicon-30 makes up 3.1% of all silicon. What is the
atomic mass of silicon?
5. Iron has four isotopes. Iron-54 has 5.8% abundance. Iron-56 has and abundance of
91.7%. Iron-57 has an abundance of 2.2% and Iron-58 has an abundance of 0.3%.
What is the atomic mass of iron?
6. Nickel has five isotopes. Nickel-58 has an abundance of 68%. Nickel-60 has an
abundance of 26%. Nickel-61 has an abundance of 1%. Nickel-62 has abundance
of 4% and Nickel-64 has an abundance of 1%. What is the atomic mass of nickel?