1. General, Organic, and Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 3
Atoms and Elements
3.5
Isotopes and
Atomic Mass
Lectures
© 2013 Pearson Education, Inc.

2. Isotopes
Isotopes
are atoms of the same element that have different mass numbers.
have the same number of protons but different numbers of neutrons.
can be distinguished by atomic symbols.

3. Isotopes and Mass and Atomic Symbols
Since each isotope of an element has a different
number of neutrons, each isotope’s mass number will
be different. We write these as atomic symbols:
Mass numbers are in the upper left corner.
Atomic numbers are in the lower left corner.
Example: An atom of sodium with atomic number 11
and a mass number 23 has the following atomic
symbol:
mass number
atomic number 23 Na 11

4. Atomic Symbols
For an atom, the atomic symbol gives the number of
protons (p+),
neutrons (n), and
electrons (e–).
8 p+ 15 p+ 30 p+
8 n 16 n 35 n
8 e– 15 e– 30 e– 16 8 O 31 15 P 65 30 Zn

5. Learning Check
Naturally occurring carbon consists of three isotopes: C-12, C-13, and C-14. State the number of protons,
neutrons, and electrons in each of the following.
Protons ______ ______ ______
Neutrons ______ ______ ______
Electrons ______ ______ ______ 12 6 C 13 6 C 14 6 C

6. Solution
Protons 6 p+ 6 p+ 6 p+ Neutrons 6 n 7 n 8 n Electrons 6 e– 6 e– 6 e– 12 6 C 13 6 C 14 6 C

7. Learning Check
Write the nuclear symbols for atoms with the following subatomic particles: 1. 9 p+, 10 n, 9 e– ___________ 2. 17p+, 20 n, 17 e– ___________ 3. 47p+, 60 n, 47 e– ___________

8. Solution F Cl Ag Write the nuclear symbols for atoms with the
following subatomic particles:
1. 9 p+, 10 n, 9 e–
2. 17p+, 20 n, 17 e–
3. 47p+, 60 n, 47 e– 19 9 F 37 17 Cl
107 47 Ag

9. Learning Check
1. Which of the following pairs are isotopes of the same element? 2. In which of the following pairs do both atoms have 8 neutrons? A. B. C. 12 X 15 X 6 7 12 Z 14 Z 6 6 15 Z 16 Z 7 8

10. Solution
1. Which of the following pairs are isotopes of the same element? B. Same number of protons, different number of neutrons. 2. In which of the following pairs do both atoms have 8 neutrons? 12 Z 14 Z 6 6

11. Isotopes of Magnesium

12. Isotopes of Sulfur S S S S
A sample of naturally occurring sulfur contains
several isotopes with the following abundances:
Isotope % Abundance
95.02
0.75
4.21
0.02 32 16 S 33 16 S 34 16 S 36 16 S

13. Atomic Mass Na 22.99 The atomic mass of an element
is listed below the symbol of each element on the periodic table.
gives the mass of an “average” atom of each element compared to C-12.
is not the same as the mass number.
is calculated using a weighted average. Na
22.99

14. Some Elements and Their Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.

15. Atomic Mass for Cl The atomic mass of chlorine is
based on all naturally occurring Cl isotopes.
not a whole number.
the weighted average
of the Cl-35 and Cl-37
isotopes.

16. Calculating the Atomic Mass for Cl
To calculate the atomic mass of an element, we need to know the percent abundance of each isotope and its mass.

17. Atomic Mass for Mg The atomic mass of Mg is
based on all naturally occurring Mg isotopes.
not a whole number.
the weighted average
of the Mg-24, Mg-25, and
Mg-26 isotopes.

18. Calculating Atomic Mass for Mg
To calculate the atomic mass of Mg, we use a weighted average because the atomic mass is calculated from the percent abundance of each isotope and its mass.

19. Learning Check
Using the periodic table, give the atomic mass for each of the following elements. 1. calcium __________ 2. aluminum __________ 3. lead __________ 4. barium __________ 5. iron __________

20. Solution
Using the periodic table, give the atomic mass for each of the following elements. 1. calcium amu 2. aluminum amu 3. lead amu 4. barium amu 5. iron amu

21. Learning Check
The atomic mass of chlorine is amu. Based on this information, which of the following isotopes of Cl is present in the largest amount? A. Cl-35 B. Cl-37

22. Solution
The atomic mass of chlorine is amu. Based on this information, which of the following isotopes of Cl is present in the largest amount? Solution: A. Cl-35 Since amu is very close to the mass number of 35, the isotope Cl-35 is the most prevalent isotope in a naturally occurring sample of chlorine atoms.