Worksheet on Average Atomic Mass
Name:________________
Period:_________
Date:_________
Note: amu stands for atomic mass unit, which is a concept very similar to the atomic mass
number but differing slightly. It is a unit for mass of tiny subatomic particles such as
protons and neutrons.
1. Given the data in table shown below, calculate the atomic mass of unknown element X.
Then, identify the unknown element, which is used medically to treat some mental
disorders.
Isotope
6
X
X
7
Mass
(amu)
6.015
7.016
Percent
abundance
7.5%
92.5%
2. Boron has two naturally occurring isotopes: boron-10 (abundance = 19.8%, mass =
10.013 amu); boron-11 (abundance = 80.2%, mass = 11.009 amu). Calculate the atomic
mass of boron.
3. Helium has two naturally occurring isotopes, helium-3 and helium-4. The atomic mass
of helium is 3.895 amu. Which isotope is more abundant in nature? Explain.
4. Calculate the atomic mass of magnesium. The three magnesium isotopes have atomic
mass and relative abundance of 23.985 amu (78.99%), 24.986 amu (10.00%), and 25.982
amu (11.01%).