1. Chapter 3 Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
Copyright © 2009 by 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.

3. Atomic Symbol An atomic symbol
represents a particular atom of an element.
gives the mass number in the upper left corner and the atomic number 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 23 Na
atomic number

4. Information from Atomic Symbols
The atomic symbol for a specific atom of an element
gives the
number of protons (p+),
number of neutrons (n),
and number of electrons (e-). 4

5. Information from Atomic Symbols
Examples of number of subatomic particles for atoms
Atomic symbol
O P Zn
8 p+ 15 p p+
8 n 16 n n
8 e- 15 e e-

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

7. Solution 12C 13C 14C 6 6 6 protons 6 p+ 6 p+ 6 p+ neutrons 6 n 7 n 8 n
protons 6 p+ 6 p p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-

8. Learning Check
Write the atomic symbols for atoms with the following subatomic particles:
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________

9. Solution A. 8 p+, 8 n, 8 e- 16O 8 B. 17p+, 20 n, 17e- 37Cl 17
C. 47p+, 60 n, 47 e- 107Ag 47

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

11. Solution
B. 12X 14X
The atomic symbols in “B.” represent isotopes of carbon with 6 protons each, but one has 6 neutrons and the other has 8.
C. 15X 16X
These isotopes of nitrogen and oxygen have 8 neutrons.

12. Isotopes of Magnesium

13. Isotopes of Magnesium

14. Copyright © 2009 by Pearson Education, Inc.
Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
33S
34S
36S
32S, 33S, 34S, 36S
Copyright © 2009 by Pearson Education, Inc.

15. 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 12C.
is not the same as the mass number. Na
22.99

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

17. Atomic Mass for Cl The atomic mass of chlorine is
due to all the Cl isotopes.
not a whole number.
the average of two isotopes: 35Cl and 37Cl.

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. Calculating Atomic Mass
The calculation for atomic mass requires the
percent(%) abundance of each isotope.
atomic mass of each isotope of that element.
sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) +

21. Calculating Atomic Mass for Cl
35Cl has atomic mass amu (75.76%) and 37C
has atomic mass amu (24.24%).
Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average.
x = amu
100
x = amu
Sum is atomic mass of Cl amu

22. Calculating Atomic Mass Mg
Isotope Mass Abundance
24Mg = amu x /100 = amu
25Mg = amu x /100 = amu
26Mg = amu x /100 = amu
Atomic mass (average mass) Mg = amu

23. Atomic Mass of Magnesium
The atomic mass of Mg
is due to all the Mg isotopes.
is a weighted average.
is not a whole number.

24. Learning Check
Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass ) atoms and 39.90% of 71Ga (atomic mass ) atoms.
What is the atomic mass of gallium?

25. Solution 69Ga 68.926 amu x 60.10 = 41.42 amu (from 69Ga) 100 71Ga
Atomic mass Ga = amu 31 Ga
69.72