ZURICH INTERNATIONAL SCHOOL
Coordinated Science I
Name__________________________________
Date_______________
Notes: Isotopes and Average Atomic Mass
I. Isotopes
Fill in the table below for the three atoms shown.
Number of p+
Number of n0
Number of e–
Atomic Number
Mass Number
Element
Y
Z

X
Using the example above, compare and contrast the different isotopes of the same element.
o Similarities
○ Differences

If the above is showing an example of isotopes, then what is the definition of the term?
o

Most elements have isotopes. For example, calcium has six, but chlorine has only two.
Practice: Let's examine two of the isotopes of carbon: 126C and 146C.
o What is the atomic number of carbon? _____
o How many protons does each atom have? _____
o How many neutrons does each atom have? _____
o How many electrons does each atom have? _____
II. Radioactive Isotopes

Some isotopes, such as carbon-14, have a nucleus that is unstable. These are called ______________________.
o Sooner or later the unstable atom breaks down or decays.
o A decaying atom gives out radiation in the form of rays and particles plus a
LARGE amount of energy is also emitted.


The radiation given off by radioactive isotopes (radioisotopes) can be harmful and cause radiation sickness
in large doses.
o For example, a disaster in 1968 at a nuclear
power plant in Chernobyl, Ukraine, led to the
evacuation of the area. It is uninhabitable and
there have been a large number of birth defects
and health issues in the humans and other
organisms living in the area.
But radioisotopes are also useful--sometimes we find a
use for the energy or rays/particles released. Make a list,
as you watch this video: https://youtu.be/4JkkyGLRszI
III. Relative (Average) Atomic Mass

In nature, most elements occur as a mixture of two or more isotopes. So if 11H, 21H, and 31H are all hydrogen,
which of the three masses should go on the periodic table as THE mass of hydrogen?

Relative (average) atomic mass of an element: the weighted average of the mass numbers of all naturally
occurring isotopes of that element.
o What is a weighted average?
o
An example! Imagine an apartment complex that has one-bedroom
apartments for 1000 CHF a month and two-bedroom apartments for 2000 CHF
a month. What would you say is the average cost of an apartment in the
building?
o
What if, though, 90% of the apartments are one-bedroom and 10% are two-bedroom. What would
you say is the average cost of an apartment in the building?
Apartment

Percent
Abundance
Totals
Cost
One-bedroom
X
=
Two-bedroom
X
=
+
=
When finding the relative atomic mass of an element, we do a very similar calculation in order to take into
account the different isotopes of the element and their abundances.

Below are the two isotopes of chlorine. What is the relative atomic mass of chlorine?
About 75.76% of
chlorine that exists in
nature is chlorine-35
About 24.24% of
chlorine that exists in
nature is chlorine-37
Isotope

Percent
Abundance
Totals
Mass Number
X
=
X
=
+
=
Check your answer with the relative atomic mass reported in the periodic table. Does your answer make
sense?
Practice!
1. There are three isotopes of silicon: Si-28 (92%), Si-29 (5%), and Si-30 (3%). Find silicon’s relative atomic mass.

Make your table and show your work.

Check your answer with the periodic table.
2. What is the relative atomic mass of copper? _________________ amu Based on this information, which of
copper's two isotopes is more abundant (more stable): Cu-63 or Cu-65? ____________ How do you know?
3. What is the difference between mass number and relative atomic mass?
4. MORE practice! Determine the average atomic mass of the following mixtures of isotopes. Make sure to clearly
show your work for each.
a. 80% I-127, 17% I-126, and 3% I-128
b. 50% Au-197, 50% Au-198
c. 15% Fe-55, 85% Fe-56
d. 99% H-1, 0.8% H-2, 0.2% H-3
e. 95% N-14, 3% N-15, 2% N-16
f. 98% C-12, 2% C-14