1. Chapter 4 Atoms and Elements
4.5
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
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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 11

4. Information from Nuclear Symbols
The nuclear symbol indicates 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 35 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
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 6 p+ 6 p+ 6 p+
neutrons 6 n 7 n 8 n
electrons 6 e- 6 e- 6 e-

7. Learning Check A. 8 p+, 8 n, 8 e- ___________
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
Write the nuclear symbols for atoms with the following subatomic particles:
16O
A. 8 p+, 8 n, 8 e
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
7 8

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

11. 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.

12. Isotopes of Magnesium
In naturally occurring magnesium, there are three isotopes.
Table 4.7
24Mg 25Mg 26Mg
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13. Examples of Isotopes and Their Atomic Masses
Most elements have two or more isotopes that contribute to the atomic mass of that element.
Table 4.8
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Publishing as Benjamin Cummings

14. Calculating Average Weight of “da pineapple”
The calculation for the average weight of a pineapple requires
Percent(%) abundance of each kine of pineapple.
Atomic mass of each isotope of that element.
Sum of the weighted averages.
mass isotope(1) x (%) + mass isotope(2) x (%) + …

15. 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 isotope(1) x (%) + mass isotope(2) x (%) + …

16. 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.
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Publishing as Benjamin Cummings

17. Calculating Atomic Mass
Isotope Mass Abundance
24Mg = amu x /100 = amu
25Mg = amu x /100 = amu
26Mg = amu x /100 = amu
Atomic mass (average mass) Mg = amu 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 for Cl
The atomic mass of chlorine is the weighted average of two isotopes 35Cl and 37Cl.
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Publishing as Benjamin Cummings

21. Calculating Atomic Mass for Cl
35Cl has atomic mass (75.78%) and 37Cl has
atomic mass (24.22%).
The atomic mass and percent of each isotope are used to calculate the contribution of each isotope to the weighted average.
x = amu
100
x = amu
amu
The sum is the weighted average or 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 (atomic mass 68.93) atoms and 39.89% of 71Ga (atomic mass 70.93) atoms.
What is the atomic mass of gallium?

23. Solution 69Ga 68.93 amu x 60.11 = 41.43 amu (from 69Ga) 100 71Ga
Atomic mass Ga = amu 31 Ga
69.72