1. Chapter 4 Atoms and Elements
4.5 Isotopes and Atomic Mass
Learning Goal Give the number of protons, electrons, and neutrons in one or more of the isotopes of an element; calculate the atomic mass of an element using the percent abundance and mass of its naturally occurring isotopes.
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2. are atoms of the same element that have different mass numbers
Isotopes
Isotopes
are atoms of the same element that have different mass numbers
have the same number of protons, but different numbers of neutrons
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3. represents a particular isotope of an element
Atomic Symbols
An atomic symbol
represents a particular isotope of an element
gives the mass number in the upper left corner and the atomic number in the lower left corner
mass number
Chemical Symbol
atomic
number

4. Atomic Symbols, Subatomic Particles
The atomic symbol indicates the number of protons, neutrons and, electrons in a specific isotope of an element.
8 protons
15 protons
30 protons
8 neutrons
16 neutrons
35 neutrons
8 electrons
15 electrons
30 electrons

5. Learning Check
Carbon consists of three naturally occurring isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these isotopes.

6. Solution
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these isotopes.
6 protons
6 neutrons
7 neutrons
8 neutrons
6 electrons

7. Learning Check
Write the atomic symbols for atoms with the following subatomic particles:
A. 8 protons, 8 neutrons, 8 electrons ____
B. 17 protons, 20 neutrons, 17 electrons ____
C. 47 protons, 60 neutrons, 47 electrons ____

8. Solution
Write the atomic symbols for atoms with the following subatomic particles:
A. 8 protons, 8 neutrons, 8 electrons
B. 17 protons, 20 neutrons, 17 electrons
C. 47 protons, 60 neutrons, 47 electron

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.

10. Solution
1. Which of the following pairs are isotopes of the same element? B.
Each has 6 protons and, the element is carbon. Different mass numbers indicate that each species has a different number of neutrons. These are isotopes.

11. Solution
2. In which of the following pairs do both atoms have 8 neutrons? C.
The number of neutrons is calculated.
mass number – atomic number = number neutrons
15 (mass number) – 7 (atomic number)
= 8 neutrons
16 (mass number) – 8 (atomic number)

12. 11 Na 22.99 Atomic Mass The atomic mass of an element is
listed below the symbol of each element on the periodic table
calculated based on the weighted average of all naturally occurring isotopes
based on its comparison
to the mass of 12C
not the same as
the mass number 11 Na
22.99
Atomic Mass

13. Isotopes and Atomic Mass
Most elements have two or more isotopes that contribute to the atomic mass of that element.

14. Isotopes of Magnesium
Magnesium has three naturally occurring isotopes.

15. Calculating Atomic Mass
The calculation for the atomic mass requires the
percent(%) abundance of each isotope
atomic mass of each isotope of that element
To calculate atomic mass of an element with 2 isotopes:
mass isotope(1) × (%)/100
+ mass isotope(2) × (%)/100
Atomic mass of element

16. Calculating Atomic Mass
The atomic mass of Mg
is a weighted average
from all the Mg isotopes
is not a whole number

17. Calculating Atomic Mass
Isotope Mass Abundance
24Mg = amu × 78.70/100 = amu
25Mg = amu × 10.13/100 = amu
26Mg = amu × 11.17/ = amu
Atomic mass of Mg = amu 12 Mg
24.31

18. Learning Check
Using the periodic table, specify the atomic mass of each element:
A. calcium __________
B. aluminum __________
C. lead __________
D. barium __________
E. iron __________

19. Solution
Using the periodic table, specify the atomic mass of each element:
A. calcium amu
B. aluminum amu
C. lead amu
D. barium amu
E. iron amu

20. Atomic Mass Chlorine The atomic mass of chlorine
is the weighted average of
two isotopes 35Cl and 37Cl.
Chlorine, with two naturally occurring isotopes, has an atomic mass of

21. Calculating Atomic Mass of Chlorine
35Cl has a mass of amu (75.76%), and 37C has a mass of amu (24.24%).
Use the atomic mass and percent of each isotope to calculate the weighted average.
34.97 amu × / = amu
35.97 amu × 24.24/ = amu
Atomic mass of Cl amu

22. Learning Check
Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.11% of 69Ga (68.93 amu) atoms and 39.89% of 71Ga (70.92 amu) atoms.
Given the abundance and mass of these isotopes, calculate the atomic mass of gallium.

23. Solution
Given the abundance and mass of these isotopes, calculate the atomic mass of gallium.
69Ga amu × 60.11/100 = amu
71Ga amu × 39.89/100 = amu
Atomic mass Ga = amu

24. Concept Map