Introducing Isotopes
Pre-thinking questions
1. How are atoms of the same element the
same?
2. What do the symbols A and Z represent?
3. How is an average calculated?
4. How would an average be calculated for
a course in which the tests count more
than the homework?
How are atoms the same?
 Atoms
 An
of the same element are the same
element is identified by its atomic
number, Z, which also represents the
number of protons in the nucleus
How can atoms of the same element be
different?

Isotopes are atoms of the same element with
different numbers of neutrons.
If the neutrons are different, what else will
change for the given atoms?
 Mass number (A)

Example of Isotopes
Example
Protons
Neutrons
Carbon-12
6
6
Carbon-13
6
7
Carbon-14
6
8
Example of Isotopes
Example
Iron-55
Iron-56
Iron-57
Protons
Neutrons
Isotope Notation

Isotopes are written with the name followed
by the isotope mass

Carbon-12 or carbon-13 or C-14
Average Atomic Mass






Atomic mass is the weighted average mass of the
known isotopes
Weighted average takes into account the mass and
the relative abundance
Carbon-12 98.90 %
Carbon-13 1.10 %
Carbon-14 trace
Average: 12.0111
Calculating a weighted average:
a classroom example

Sammy is in a college biology class that has a weighted
average. His exams count for 50% of his grades, labs are
30%, and his final counts for 20%. By the end of the
semester these are his grades:
Exams: 87%
Labs: 75%
Final: 74%

What is his overall average for the course?



Calculating Weighted Average
Avg x abundance + avg x abundance + ….
100
(50 x 87) + (30 x 75) + (20 x 74)
100
= 80.8 %
(If you calculated his grade just using the grades he would
have a 79% (87+75+74)/3
Post-thinking questions
1.
2.
3.
4.
What does the number represent in the isotope
platinum-194? Write the symbol for this atom using the
A and Z notation (superscripts and subscripts).
What is the difference between the mass number and
the atomic number of an atom?
How is an average mass different from a weighted
average mass?
The four isotopes of lead are shown below, each with
its percent by mass abundance and the composition of
its nucleus. Using these data, calculate the approximate
atomic mass of lead.
82 p
122 n
1.4%
82 p
124 n
24.1%
82 p
125 n
22.1%
82 p
126 n
52.4%