1. Chapter 3 Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

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. Nuclear Symbol A nuclear 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 Nuclear Symbols
From the nuclear symbol, we can determine the number of protons (p+), neutrons, (n), and electrons (e-) in a particular atom.
O P Zn
8 p+ 15 p p+
8 n 16 n n
8 e- 15 e e-

5. 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 ______ ______ ______

6. 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-

7. Learning Check
Write the nuclear 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- ___________

8. 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

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. 15X 15X
B. 12X 14X
C. 15X 16X

10. Solution
B. 12X 14X
Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.
C. 15X 16X
An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.

11. Isotopes of Magnesium In naturally occurring magnesium,
there are three isotopes.
Isotopes of Mg
24Mg 25Mg 26Mg

12. 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 © by Pearson Education, Inc.
Publishing as Benjamin Cummings

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

14. Isotopes of Some Elements and Their Atomic Mass
Most elements have two or more isotopes that contribute to the atomic mass of that element.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

15. 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.
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

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

17. 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